I didn't try to execute the DLL, but I doubt that checking mVersion is the problem for 2 reasons. 1 is that I didn't see any error message string related to mIRC's version inside the dll. 2 is that the dll appears to be compiled in 2007 when the active mIRC version was 6.3something, so I doubt they would design the dll to work up through future version 7.63
The url in the post above isn't a help, because they uploaded the dll yesterday without source code or indication where they got it from.
One cause of dll failure can be the OS blocking the file because "comes from another computer". But this isn't likely unless the antivirus is messing with it, or if the dll was downloaded as the DLL itself, without being inside a ZIP, and the OP didn't give any indication they did anything except upgrade.
Would also be helpful if OP says exactly what was the problem. Was there an error where the DLL couldn't be executed? Or just nothing happens? etc.