I am having the same problem, and am on Yosemite 10.10.2. I asked my husband to try logging into forums.adobe.com through his Linux-based computer, via Firefox (using my login credentials), and we got the same redirect error. Though in Googling "adobe sign in redirect loop" it appears many people across Mac platforms have this error on multiple browsers.
I have cleared my cache, cookies, everything, multiple times. Have tried typing in all caps (as the linked Behance login is in all caps) and tried unchecking the "remember me" button, but all to no avail. I am completely unable to log into any part of the forums.adobe.com sites. I can get as far as the log in page, right after clearing browser history, but as soon as I attempt to login, I get stuck in a redirect loop! I can login to Behance with these same linked credentials, no problem.
So I don't think it is a computer or a browser thing. I think it is horrible website coding on the account of Adobe.