Nope no idea... I just fixed this bug in the installer and it's the first time I've coded something in Plus! since June... it's good to be back
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Note: the setup you have currently should always work except if it's named "msgplus.exe", "msgplus1.exe" or "msgplus2.exe". Any other name (including "msgplus3.exe" for example") should work as fine as the default one. However, as I just said, it has been fixed for version 3.10.