As an alternative to working out a fix, Ancestry could roll back to a version of their server software that worked with IE10.
FTM 2011 with IE10 with the latest IE patch (installed on 10th April) was working for me on 18th April, but wasn't on the 19th April. There were no IE 10 updates performed in between, so something must have changed at the Ancestry server end.
As part of any software upgrade process there should be a rollback procedure if things go wrong. Maybe it is time to do the rollback!
I don't think it is acceptable to have to downgrade to IE9 to get FTM to work, and this isn't possible for Windows 8 users anyway.