Workaround specifically for people who log in to a number of different company files with different identities:
I had the same issue, the workaround for me was to create a new user on one account that has the same email address as the one on the other account and send an invite. Now when I log in with this new universal identity I can open both of my company files as before. Probably slightly more efficient now as I only log in once to access both files. I hope this helps a few people; I was ready to cancel MYOB altogether yesterday.