Hi!
First of all - thanks for excellent plugin!
Come back to the subject.
I found this on my test environment, which I create by copying of my production environment (MyBB 1.6.8).
In my database, for some reasons, field mybb_adminoptions.permissions are empty for Administrators group members (except user with uid=0). MyBB are happy with that and use inheritance from group permissions.
During installation XThreads do unserialisation from mybb_adminoptions.permissions, add permission to manage additional fields and serialize back.
As result - all admins with uid>0 lost any Administrator permissions except permission to manage additional fields.
Why this field are empty ? Probably because my forum starts with phpbb (very old and very customized version), then, some years ago, it was semi-manually migrated to MyBB 1.4.х, then it was upgraded to MyBB 1.6.x. All admins are admins already from phpbb's times.
I'll suggest to add some check for mybb_adminoptions.permissions content in installation procedure. Or remove automated permission addition at all and leave related message in readme to add this permission manually.
thanks again.