My problems (an identical error occurs with either of my two large files) occur when I try to upload a file from FTM to Ancestry. The error msg is, "An error has occurred during the data upload to Ancestry." Obviously, not particularly helpful and still no word from Ancestry regarding the file I uploaded weeks ago.
Interesting (at least to me but not to the developers at Ancestry as I mentioned this problem as well) when I try to do an export of this file in Gedcom 5.5 I get an error msg stating, "The export process failed because an unknown error occurred." However, I can successfully Backup and Restore these files as well as export in FTM format.
All this leads to the obvious conclusion that my files are corrupt in some fashion. However, with in excess of 7,000 individuals in each file, there doesn't seem to be any easy way to discover what individuals (if, in fact, it's a problem with tree structure, e.g. circular families.)
I did suggest to the Help desk that an error detection routine in FTM would be a tool to help resolve this problem.
Their answer was that this function had been often requested but could give no answer when, or indeed if, this capability would be released. So I request that as you talk to the Help desk you also request this ability.