Use a better version
Rude! I thought you knew better than that. 
If he wants to use 4.6, that’s him. If you want to use the LATEST AND GREATEST version, that’s you. But don’t get all up and arms about it. 
P.S.: I have MSN Messenger 4.6 installed alongside WLM 8.5. You can kill me now, if you want. 
Welp, someone got salty and flagged my post. Someone actually used that feature. Wow.
Windows Messenger 4.6.0082 signs in fine on Windows 8.1 for me, using the reg file on the escargot site.
What versions are you using? What method did you use to revive Windows Messenger?
reg file too
the same poblem
As a next step, you could try making another Windows user account, logging into that account and try signing in from there. That will at least indicate if it’s a problem with your specific user or the entire Windows install.
If it’s the entire Windows install, I believe debug symbols are available for 4.6, so if you desire to go further, you can download procdump to a folder of your choice, then run it from the command line with:
procdump -w -e msmsgs.exe
Then start Windows Messenger until it fails, procdump will create a .dmp file that you can zip and attach to a reply to be analyzed.