The Maintenance page mentions having the file ../mcomix-other/unrar/UnRAR64.dll7 for compilation of distribution files, but the installer script actually seems to look for ../mcomix-other/unrar/UnRar64.dll (please notice both the case and the file extension discrepancy).
Thank you for reporting this, there were indeed some inconsistencies here. The upstream DLL is named UnRAR64.dll, while MComix used UnRar64.dll internally to find the library. In addition, there was a "7" appended on the Maintenance wiki page, which was a typo. I consolidated every occurance, now only UnRAR64.dll should be used everywhere.
In practice, this shouldn't make much of a difference, since Windows file systems are case insensitive.