Is it possible that you are using a script that is interfering in some way?
It's a script indeed, it's the nick coloring one, sorry about the trouble and I will talk to maroon about this issue.
Damn it, what a waste of your time Khaled, sorry about this, the same nickcolor script (and the other one, crc vs md5) locks the playback buffer timestamp as well. Unloaded the script and everything runs perfectly.
Will we see an option to randomly color the nicks (and keep the same color for each nick) and an option to gray out away users by default from mIRC?