I decided to be a little brave and try to cancel a couple of award reservations (made with PMUA) and amend another online.

In all cases I received error messages at the very last step, after I hit "confirm". However, both cancelled reservations no longer show up in My Reservations, and the the reservation that I was trying to amend showed up with the amended date and flight (and I was able to go in an assign myself a seat).

As far as I can tell, the cancellations and amends all worked and the error messages were spurious.

UA Insider - will PM you will confirmation numbers etc in case you wanted to investigate further.