Update: I increased the memory limit in wp-config to 2GB, rolled back to version 5.6.10 then ran the database upgrade, and it worked. Just increasing the memory limit didn’t work.
I upgraded the plugin to the latest version after that, and everything is ok. Custom fields are back in the WordPress backend.
I heard back from support. They suggested switching themes, but my site’s custom theme has to be kept online. They also suggested disabling Jetpack, but it hasn’t fixed the problem.
I’m going to try increasing the memory limit again. If it doesn’t work, I’ll just downgrade. Here are the instructions if anyone needs them. Scroll down to Troubleshooting.
https://www.advancedcustomfields.com/resources/upgrade-guide-version-5/
I have the same problem, and all custom fields are gone from the WordPress backend. The fields are still working on the website though. I disabled all other plugins, increased the memory limit, and no there were no debug errors found. The upgrade still won’t complete.
I completed the database upgrade on another site in a few seconds. I haven’t heard back from support on this for a few days.
