User registration issues

Hi, Over the last few weeks I have received notification from my client that their site seems to ‘occasionally’ have issues when a new customer tries to register an account.

I am finding it hard to work out what the issue could be, because I can’t seem to get very good information out of the client as to what the error is.
The information that I am given is that after the user tries to enter their password and then hit submit, the form doesn’t do anything or the page refreshes or sometimes they have said the form redirects to the login page (but then the user would try to enter their username and password, but still would not let them in – this is because the registration did not work).

I have not been able to ever replicate the issue myself, so I’m having a hard time trying to work out a solution.

I have just recently asked my host to exclude the registration page from the cache. So I am hoping this may help.Just wondering if you would have any other ideas though in case it doesn’t fix it? I cannot try the usual plugin/theme conflict test in this case as it only seems to happen to a few customers and as I have said earlier I have not been able to replicate the issue.

When I try and look at my php error logs for around the time the user has tried to register. I see these errors:
WARNING: child 4656 said into stderr: “NOTICE: PHP message: PHP Warning: preg_match() expects parameter 2 to be string, array given in /wordpress/wp-includes/class-wp-block-parser.php on line 417”
WARNING: child 4656 said into stderr: “NOTICE: PHP message: PHP Warning: strlen() expects parameter 1 to be string, array given in /wordpress/wp-includes/class-wp-block-parser.php on line 489”

I’m not sure if your plugin calls upon these files at any point.

My site also only allows user to create an account that have received a link with a unique key at the end of it. But this just pre-fills the email field (which I have made this field un-editable) – so the only thing that would normally prevent this form from continuing is if the email field was blank, but it wouldn’t be blank if they followed the link in their email.

Any help in trying to find a solution will be greatly appreciated.