Having issues with recording new payments for registrations paid with credit cards
I don't have the same error, but a similar issue with workflow for processing additional cc registrations where we can't "submit a credit card edit to an event registration" to add these items for folks. That would be ideal.

Last year we worked around by either having them run through registration again for only the additional item or submitting a credit card registration for the additional item in the back end, then consolidating the line items in one record, updating the amount paid, forcing the status 'registered' at the top, and deleting the second event registration.

This caused a bunch of problems for example the event report dollar amount was off because when editing event registration line items you can't add in discount codes or edit the event total, only the contribution total. Therefore only the contributions dollar amount with that financial code was correct.

In the end the event ran and the badges printed. Not a great answer but this is what we ad-hoc threw together and wanted to say you aren't the only one who has had this workflow issue for events.