Multiple upload fields AJAX – “undefined” error at 0%

  • Resolved alalondon

    (@alalondon)


    1 week, 5 days ago

    Plugin version: Forminator 1.55.0 (free)
    WordPress version: 7.0
    Theme: Divi
    Hosting: Oderland (LiteSpeed server, shared hosting)

    Problem description:

    We have a paginated form (3 pages) with three file upload fields, all set to AJAX upload method:

    • upload-1: ONE main picture (required, 1 file, 2MB limit)
    • upload-2: 6-7 gallery pictures (required, up to 8 files, 2MB limit)
    • upload-3: Workshop pictures (optional)

    We are experiencing an intermittent issue specifically with the AJAX file upload fields. When a user selects a file to upload, the upload appears to start but immediately shows “undefined” in red below the upload field, with the progress stuck at 0%. This prevents the user from completing the upload and submitting the form. Users typically retry 5-10 times before either succeeding or giving up – the issue is intermittent and the same user can succeed on a later attempt without changing anything.

    Evidence from server access logs (June 22, 2026):

    We have analyzed our SSL access logs and found a consistent pattern across multiple users and browsers:

    • 10:38–10:50 – Windows/Edge: 7 failed AJAX attempts → finally succeeded at 10:50
    • 11:01–11:15 – Mac/Safari 15.4: 8 failed AJAX attempts → finally succeeded at 11:15
    • 14:15 – iPhone/Safari iOS 18.7: failed AJAX call → no successful submit
    • 20:03–20:11 – Windows/Chrome: 7 failed attempts → succeeded at 20:11
    • 20:05 – Windows/Firefox: failed AJAX call → no successful submit
    • 20:05 – Mac/Chrome: failed AJAX call → no successful submit

    The pattern is the same across all cases: repeated POST to wp-admin/admin-ajax.php with no successful submission, until the user eventually retries enough times to get through. The issue is not browser-specific – we see it on Safari (iOS and macOS), Chrome (Windows and macOS), Edge (Windows) and Firefox (Windows).

    What we have already tried:

    • Disabled Honeypot protection
    • Changed gallery upload field from “On form submission” to AJAX
    • Excluded form pages from LiteSpeed cache
    • Verified PHP settings (max_execution_time=90, post_max_size=50M, memory_limit=256M – all fine)
    • Confirmed no relevant PHP errors in debug.log during failed submissions

    Relevant note: We found this older fix from a WPMU DEV developer (Thobk, Jira SLS-2280) for a multiple upload field conflict: https://gist.github.com/wpmudev-sls/2a2da7f5832acb9577325bdbb2295f66 – but it is from 2021 and we are unsure if it is still valid for Forminator 1.55.0.

    Questions:

    1. Is this a known issue in Forminator 1.55.0 with multiple AJAX upload fields?
    2. Is the 2021 fix still applicable, or has it been addressed in a recent version?
    3. What does the “undefined” error at 0% indicate specifically – is it a JavaScript conflict, a server-side issue, or something else?

    Thank you.

    • This topic was modified 1 week, 5 days ago by alalondon.
Viewing 6 replies - 1 through 6 (of 6 total)
  • Plugin Support Nebu John – WPMU DEV Support

    (@wpmudevsupport14)

    1 week, 5 days ago

    Hi @alalondon,

    We are sorry to know that you are experiencing an issue using our plugin, and thank you for reaching out to us.

    Let me assure you that we haven’t received any similar reports at our end.

    The fix that you shared is specifically for Safari, but I see that you were able to replicate the issue on different browsers and different devices. Also, the code might not be helpful, as Forminator has had quite a few updates released since 2021.

    I tried to reproduce the issue on a similar setup as you mentioned, but I wasn’t able to replicate it. It appears the issue is specific to your setup or form.

    To investigate the issue further, could you please share an export of the form using Google Drive or Pastebin.com in your next response?

    I hope the following guide comes in handy: https://wpmudev.com/docs/wpmu-dev-plugins/forminator/#import-export

    Looking forward to hearing back from you.

    Best Regards,
    Nebu John

    Thread Starter alalondon

    (@alalondon)

    1 week, 4 days ago

    Hi, please see https://pastebin.com/rqEk8yQS.

    Best regards,

    Mattias

    Plugin Support Imran – WPMU DEV Support

    (@wpmudev-support9)

    1 week, 3 days ago

    Hello @alalondon,

    I hope things are going well for you.

    I’ve created a testing environment with:

    Plugin version: Forminator 1.55.0 (free)
WordPress version: 7.0
Theme: Divi Version: 5.8.0
PHP version: 8.5.3 (Supports 64bit values)
Server architecture: Linux 7.0.11-orbstack-aarch64
Web server: LiteSpeed

    Here is the testing video: https://monosnap.ai/file/pZutTkmCbLSlngmgxS9Dph6pQGEHrX

    I couldn’t replicate the issue in the environment. It could be something with the hosting or another plugin component in that specific setup that seems to be causing the issue.

    Since WordPress offers a wide variety of plugins and themes, any of them could conflict. This requires a plugin conflict test. Let me help you with the steps:

    If your hosting offers the staging site, then please follow the entire test on the staging site; if not, then try this plugin https://wordpress.org/plugins/wp-staging/

    1. Create an entire site backup for safety
    2. Deactivate all the plugins.
    3. Deactivate the current theme and set the WordPress default theme.
    4. To ensure that all the plugins and themes are deactivated, try uploading the images to the form and check whether they’re working as expected or cause any errors during uploading. If not, then.
    5. Reactivate the theme and check whether the error reproduces. If not, keep reactivating the plugin until it produces the error.
    6. The last activated plugin will be causing the issue.

    Here is the conflict test documentation, which you can refer to if you feel stuck while doing a conflict test.
    https://wpmudev.com/docs/getting-started/getting-support/#conflict-test
    https://wpmudev.com/wp-content/uploads/2015/09/Support-Process-Support-Process.gif.

    Please perform the conflict test and share the outcome with us.

    –
    Kind Regards,
    Imran Khan

    Plugin Support Saurabh – WPMU DEV Support

    (@wpmudev-support7)

    4 days, 7 hours ago

    Hello @alalondon,

    We haven’t heard from you in a while, I’ll go and mark this thread as resolved. If you have any additional questions or require further help, please let us know!

    Kind Regards,
    Saurabh

    Thread Starter alalondon

    (@alalondon)

    4 days, 6 hours ago

    Hi,

    Thank you for your support.

    I have not been able to replicate the issue myself either and this has been the main issue from the beginning, only our customer have had the issue.

    But, at least, it seems with the Ajax file upload, it’s more stable than what we initially had upload on submit. I can see that you had some issues when you added the wrong number of files for the feature image and that you had to redo the upload. But it seems not to be the issues that we have had here.

    Hopefully, it will work better for us moving forward – at least it seems we have very few issues the last two days before submittion.

    /Mattias

    Plugin Support Saurabh – WPMU DEV Support

    (@wpmudev-support7)

    4 days, 5 hours ago

    Hello @alalondon,

    Glad to know that Ajax file upload is working in a stable manner for you.

    I can see that you had some issues when you added the wrong number of files for the feature image and that you had to redo the upload.

    Do you mean in the screen recording we shared earlier? It was the testing step to confirm that “upload-1: ONE main picture (required, 1 file, 2MB limit)” based on your issue description, it was working as expected. Please let me know in case I am missing anything, or if I am unable to understand it fully.

    In case you still notice any issues around this, please feel free to let us know.

    Kind Regards,
    Saurabh

Viewing 6 replies - 1 through 6 (of 6 total)

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