Upload Errors and No Thumbnails

We are getting error messages in the modal when uploading new images “upload failed” even though it shows the image previews and the images actually do get uploaded into the gallery folder. BUT… no thumbnails are generated, no thumbnails folder is created and they don’t appear on the gallery page – just placeholders with missing thumbs.

When trying to add these new galleries to a post and publish, it throws a 503 error. (there’s another recent thread describing the exact same issues as I’m having)
All of our old/previously uploaded galleries before the recent updates (with the same shortcodes) display just fine, so it seems to be something happening during the upload process.
Another glithc is that the shortcodes for basic galleries seem to work — but none of the Plus versions shortcodes work like filmstrip / masonry etc.
(Again, this is only with newly added galleries. Previously uploaded images before the recent update display fine in all gallery display formats.)

I tried to rollback to the old versions of NextGen 3.59 and an older version of Plus but after rolling back I’m still getting the same upload errors. Can anyone explain that? Would there be additional or different mySQL fields that were added during the upgrade or something in the new version causing this? I just don’t get after rolling back why we still can’t upload without errors. Everything else seems to work fine.

This is a mess either way and have been pulling my hair out trying to find a solution, but I’m thinking this is a developer issue and a plugin problem that needs to be fixed.