Hi,
The ownCloud QA is using the official docker images for testing which are using mod_php with prefork. And I don’t expect this to change at any point in time.
To have a reliable setup, that is also QA tested it is recommended to use mod_php with prefork.
There are many community users out there which are using ownCloud successfully on PHP-FPM, however, when you specifically run into problems after enabling PHP-FPM, the answer is to revert back to mod_php, because that is the only supported environment.
Finally like you said, it worked before, so the issue is definitely related to either your apache or PHP-FPM configuration. I would recommend to google the error message.