I suffer this problem since long (windows 98 oem), also * Error creating dialog, I wrote a script with a quite large dialog and just opening two mirc instances, no IRC connections, is already enough to not be able to open the dialog of the last started.
It's fixable, in a certain degree, by increasing that value, but the fact is that the original cause is the chosen width of a register, aging back to 1995 and never increased, also not in Vista - a rather serious middlefingerup if you take into account the immense increase of RAM memory since those days, the bigger screen resolutions and faster CPU's and thus the tendency to run more and more applications simultaneous.
In my windows 98, it's even hardcoded (not a registry setting), so I can't do a thing about it.
Drawback is that I'm unable to test with two mirc instances anymore.
So yes, blame Microsoft, not mIRC, it's just mIRC's application nature that makes it more likely to bring up the problem first.