I'm now wondering if this is related to the reason why I cannot link my Epic Games account to this website since it too was created using my @yahoo.com email address? The again, my Game2Gather and Steam accounts were both created using my @yahoo.com email address, and my Steam account linked up just fine, so it's probably just a glitch or your @yahoo.com account has already been registered with Games2Gather, which this website is built upon, so I'd first try logging in your Games2Gather account first and see what happens.

Try a password reset (Forgot my password) at the login page. If this doesn't work, then I'd try Googling why your specific @yahoo.com email account is being blocked by this website. The true cause of this might be country, I don't know for sure, but Googling might tell you the reason.