I don't know why exactly the online timer features is causing a freeze but it's a bug that was very recently (after the release of mIRC 7.51) reported and fixed!
However a beta version has been released which fix the bug, you can find it here https://forums.mirc.com/ubbthreads.php/topics/225704/mIRC_beta
Just rename your mirc.exe and then drop the beta mirc.exe into your mIRC folder.
Note: eventually don't forget to re-enable your script with /remote on if you used /remote off

#mircscripting @ irc.swiftirc.net == the best mIRC help channel