AOL bounces with: AOL will not accept delivery of this message [closed]

AOL started bouncing emails from my mail server with the following error:

521 5.2.1 :  AOL will not accept delivery of this message.
554 5.5.0 Remote protocol error

On the troubleshooting page AOL asks for specific error code but all I get is a generic message. The emails are sent from Outlook, without any attachments. I don't send out any mass email/newsletters, only personal email.

What can cause this and how should I proceed? Without the specific code I can't proceed with AOL. Have anyone had this problem?

You can try submitting the form with no error code selected, and hope for the best. If you're lucky, a human being might read it.

You can also try contacting AOL Postmaster by phone, on +1 703 265-4670 (reportedly AOL postmaster operations are now being handled in India; you've been warned).

Checking around for similar errors, and looking through AOL's Postmaster errors, nothing immediately ties up with what you've reported but doing some cross-referencing (for example from this thread), the linked error codes (521 5.2.1 which you reference in your question) are possibly linked to the 421 CON:B1 error code from the above-linked error code list from AOL, which reads:

21 CON:B1 The IP address has been tempfailed due to a spike in unfavorable e-mail statistics.

It strikes me as odd that they would have ambiguous error code reporting, but since we currently do not have very much else to go on, I suggest that you check your IP reputation using the AOL tool and follow up accordingly.