MYOB have finally responded to the outcry, and they say they are working on fixing it.
They also said this:
"We’re investigating an issue with the Bankfeed allocation service; in the meantime, you can manually match transactions as a workaround."
What it means for many of us is that you will need to manually CREATE a Spend Money or Receive Money transaction in the Banking section, selecting the appropriate account/category, and ensure it exactly matches the bank feed transaction amount and date, then you can go back to bank feeds and match the transaction to the one you manually created. This worked for me last night, but I was very glad I only had two of them to do!
It's a HUGE time-suck for all of us, so fingers crossed they fix it soon!