I had an 'error has occurred' yesterday.
I could not send an invoice 'because I was neither the seller nor the buyer of the item'

Buyer was not happy, and I wasn't either.
I just had to keep trying.
Goodness knows what was happening there.
Eventually it cleared, but I had a message from eBay telling me how to send invoices. (I had sent an email telling eBay exactly what had come up as the ERROR. No help there.