Hi Elliot, Anja,
Update.
I wanted to test if maybe a specific ACF field type was causing the error. So I put a field group back in draft status. It had only 1 text field. And right away the problem was solved. So I did put the field group in published status to check if the error was back again. But the error did not happen! until now I was still in the Twenty Thirteen theme with ACF as only plugin. I turned on all my plugins, still working! turned on my custom theme, still working!
(to be complete, I also had to put some widgets back because the were lost after theme en plugin changes)
So for me, putting a field group in draft status and publish it over again helped to solve the problem.
I hope this helps you.
Best regards,
Dennis
