Thanks for debug log. The change I made above has indeed resolved the issue.
That said, I notice that InspIRCd is using numeric 344 in the whois reply. This numeric is used as a MODE reply on some ircds, which is how mIRC has always parsed it. I have added a change so that numeric 344 is now treated as a whois reply if it contains a specific number of parameters/content. InspIRCd actually has two formats for numeric 344, one for InspIRCd 2.0 and one for InpsIRCd 3.0, so mIRC has to handle both.
These changes will be in the next beta.
