I was having some issues signing in to the ASC, and saw this thread that described my exact problem.
That thread in turn, led me to this other thread about the same issue, where user DeZYnur's post (#15) led me to a solution.
I just wanted to document (outside of the ASC) what I found in hopes that it may assist other users.
I went to https://appleid.apple.com. On the right, under the "Create an Apple ID" button, there's a link labeled Find your Apple ID. That dropped me on a page with a form asking for my name and one or more email addresses. I filled that out using the email address (i.e., Apple ID) that was giving me so much trouble. Apple will then sent me a password reset email. I was daunted by this at first, as I did not want to reset any passwords, but I went ahead and followed the link.
The resulting page showed a list of two "Apple IDs" associated with my email. One of these was the Apple ID I was having trouble using with the ASC. The other was an old username (not an email address, as used by Apple ID now, but an actual username) associated with a Apple Developer Center account I haven't used in five or six years. I guess whatever user account migration Apple has done over the years has "promoted" other usernames like these to Apple IDs. Weird.
I wen ahead and reset the password for that Developer account and then tried signing in to the ASC with it.
Bingo! I was signed in to the ASC. In my profile I could see discussions I'd participated in between 2005-2008 (which was the last time I used the ASC on regular basis).
Okay, so having confirmed that this old account was the one associated with my current Apple ID's email address (and thus preventing me from signing in to the ASC with my current Apple ID), I went back to the Apple ID management site (linked above) and changed the email address associated with this old Developer account to one different from my current Apple ID, but also under my control.
After a few minutes and some cache flushing in Safari, I was able to sign in to ASC with my current, correct Apple ID and create an ASC username. Problem solved.
I have no idea whether or not it's possible for me to contact Apple about deleting that old Developer ID, as there are some archived ASC discussions associated with it. At the least, I am now aware that the old account exists and have the login info.
Aside: I know that I used to have an Apple ID I created in 1999 when I bought my first PowerMac (and that probably has archived ASC discussions associated with it), as well as one that's associated with my now-abandoned me.com account. I wish Apple would work out a scheme that would make it easy to consolidate all of these old accounts, or at least associates my relationship with them to me the person, rather than whatever email address I have at the time. (Or maybe I just don't understand this well enough.)
Anyway, I hope this helps. It seems like a lot of people are having this issue.
That thread in turn, led me to this other thread about the same issue, where user DeZYnur's post (#15) led me to a solution.
I just wanted to document (outside of the ASC) what I found in hopes that it may assist other users.
I went to https://appleid.apple.com. On the right, under the "Create an Apple ID" button, there's a link labeled Find your Apple ID. That dropped me on a page with a form asking for my name and one or more email addresses. I filled that out using the email address (i.e., Apple ID) that was giving me so much trouble. Apple will then sent me a password reset email. I was daunted by this at first, as I did not want to reset any passwords, but I went ahead and followed the link.
The resulting page showed a list of two "Apple IDs" associated with my email. One of these was the Apple ID I was having trouble using with the ASC. The other was an old username (not an email address, as used by Apple ID now, but an actual username) associated with a Apple Developer Center account I haven't used in five or six years. I guess whatever user account migration Apple has done over the years has "promoted" other usernames like these to Apple IDs. Weird.
I wen ahead and reset the password for that Developer account and then tried signing in to the ASC with it.
Bingo! I was signed in to the ASC. In my profile I could see discussions I'd participated in between 2005-2008 (which was the last time I used the ASC on regular basis).
Okay, so having confirmed that this old account was the one associated with my current Apple ID's email address (and thus preventing me from signing in to the ASC with my current Apple ID), I went back to the Apple ID management site (linked above) and changed the email address associated with this old Developer account to one different from my current Apple ID, but also under my control.
After a few minutes and some cache flushing in Safari, I was able to sign in to ASC with my current, correct Apple ID and create an ASC username. Problem solved.
I have no idea whether or not it's possible for me to contact Apple about deleting that old Developer ID, as there are some archived ASC discussions associated with it. At the least, I am now aware that the old account exists and have the login info.
Aside: I know that I used to have an Apple ID I created in 1999 when I bought my first PowerMac (and that probably has archived ASC discussions associated with it), as well as one that's associated with my now-abandoned me.com account. I wish Apple would work out a scheme that would make it easy to consolidate all of these old accounts, or at least associates my relationship with them to me the person, rather than whatever email address I have at the time. (Or maybe I just don't understand this well enough.)
Anyway, I hope this helps. It seems like a lot of people are having this issue.