Ok, so the problem is the following:

I shut down my PC while mIRC was running, using the power switch (due to a game freezing the computer).

I rebooted and started up mIRC again, and now it had "forgotten" the files which are supposed to be loaded on start (scripts, variables, userlist, etc).

While this behaviour isn't common at all (happened to two times in ~5 years), is there a way to ensure that mIRC remembers the files to load, regardless of the way it exited? Note that this isn't a 6.17 specific problem.