Try to enter the (global, as it seems) topic from a forum,
which is
at no point marked in no posting/locked sign (circle with line on it diagonally)
on the topic "tree" you move through.
- try, for example, to enter that global topic from general discussion forum (A similar thing happened to me - I couldn't edit / delete my posts in a forum, and then I tried this and could.)
This is very difficult for me to explain,
I hope this understood still and I also hope it helps (I am not sure it will work)I guess it happens for global announcements etc? (can be entered from all forums, either locked or open to you)
Maybe if you enter it from a locked for posting forum for you, it's considered as a topic in that (locked) forum in a way, hence locked, and if you join from an open topic, you can post?