Moderator of my forum has problem, since yesterday he can't add messages, while he can delete them, edit and etc. All in all he can do everything moderator can, but not answering people. When he try to add message, a warn table appears, in Russian it says: "Ó âàñ íåò ïðàâ äëÿ âûïîëíåíèÿ ýòîé îïåðàöèè", in English it's something like: "You have now rights to do this operation".
How can be this problem solved?

And I forget tell you, I tried to answer using his (moderator's) login and password and a message was added.

We have no ideas. If you can add a message using his account, it appears to be exclusively this user's problem, and who knows where the permissions are messed up... I only know for sure this is not a miniBB problem.