Admin Failed to save entry: %s

Hi, I updated a site on grav from 1.6.x to v1.7.13 (latest) and now I get this "Admin Failed to save entry: %s" error every time when trying to save a page in the admin. Same thing when making a new page and trying to save that. It's so vague I'm not sure where to start even. Any ideas?

A good idea is to start with logs :) Maybe something there
I have a feeling it's related to file permissions. Did you check the upgrade guide?

There was nothing in the logs, but not thinking of checking the upgrade guide was a duh moment haha, thanks! Saving works without issues when I switch back to regular Grav Pages, so it seems something's not playing nice with the new Flex Objects system.

This topic might help
/forum/support/migrating-pages-to-flex-pages-t9182