Hi, apologies for not updating this!
I sent an email to the address mentioned by @jorgev with my initial message. Half an hour later I received this reply from a separate address:
can you please log-in with the email address that owns this add-on into Firefox Accounts (or create a new Firefox Account for this), in order to link your AMO account once to your Firefox Account? That should give you access to the add-on needed.
As for the disabled version, I am not sure why that happened. Please try again once you have recovered access to your account.
I am signed into Firefox with max@ABCD.com, which is the email address that owns the add-on. When I try to log in to the Addons site the page knows my email address and just asks for a password, but after entering it is when I get the Internal Server Error.
I think I initially created the addon with the email address firstname.lastname@example.org, however that email is now the secondary email address of my max@ABCD.com account so I cannot sign in with it.
Later they replied
The issue is that you have two AMO accounts now, so AMO doesn’t know which one to pick. I disconnected your old email address from your Firefox account, so you should be able to log in with your primary email address which also owns the add-on mentioned.
Should you ever need to access your old AMO account, you need to remove the secondary email in Firefox Accounts and create a separate Firefox Account using that email address.
This solved the issue for me, total time was less than two days