Hi @cerulean
The issue was a strange one, but I’ve tracked it down and fixed it.
A plugin you use (Post Expirator) is creating extra category check-boxes on the page which are being read by ACF and posted to the AJAX compare function.
I have added a fix to the JS that now makes sure the check-box it see’s is visible and not disabled.
Your version of ACF now contains this patch and it will also be included in the next version of ACF.
Cheers
E
