Several blind alleys later, I finally managed to identify the cause of the problem: a java applet with the ability to monitor the clipboard (to catch-and-process a set of specific urls on occasion), burried in my system startup routine.
Even though the actual cb monitoring option of the app was disabled for the time being, it apparantly kept reading the cb and thus, somehow, interfered with the rapid, successive cb-access attempts of mIRC.
Thanks to all who looked into the issue; sorry for the bother!