Okay, I rolled back to a 4.* version – fields where there, but this should not be a solution.
I edited wp_options → acf_version to “4.4.12” and performed a DB-update on the site that was causing the problem. Did not work.
Then I went through ALL subsites on a database level (wp_1_, wp_2_, etc.) and changed all wp_options.acf_versions to “4.4.12”, even on sites where I do not use the plugin actively. So in the end I had to update ALL of them.
I did that in one go and this did the job. My fields are back and everything works as it should.
It seems that if the update failed on one of the sites, all sites suffer. Hope, this helps you guys, too.
