[Charles_1]

Yes, I checked all checkboxes for that user for that thread:

Edit Posts, Open/Close, Edit Threads, Manage Threads, Moderate Posts, Moderate Attachments, Edit Polls.

But "Moderate Attachments" is here only for moderation purpose, which is not in use in our forum, it is also in explanation of this checkbox:

"If attachment moderation is enabled in this moderator's forum, this permission will allow this moderator to approve attachments awaiting moderation."

I tried it with 2 different users just to be sure.

If I use "User Threads", which is based on whole sections and usergroups, then indefinitely managing of attachments is working fine. but not for "Thread Admins".