Well this error comes up regardless of how many instances of mIRC are running on the box. If we have 10 instances of mIRC each running a single bot that is on 20ish channels we will see this error on all of them at some point when they attempt to join a channel. We have mutiserved 5 to 12 bots on single instances of mIRC to reduce their foot print and to ultimately reduce the amount of windows thinking that maybe the main mIRC window takes up a window slot as well. Ultimately if we made them all socket bots this wouldn't happen of course but it seems that is almost like reinventing the wheel... and a considerable amount of script would need to be changed/updated/totally reworked to accomidate for such a drastic modification.
It isn't the processing the events that is the problem here - its simply having that many channel windows open. It seems from your test that channel windows are taking up a roughly 2 slots. How those are measured I'm not really sure I really hope that there can be something done to remedy it in the future
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