Anythony,
A couple of things to try to investigate, or work around the issue?
1) Try to sync using the Synchronize Changes command (i.e. NOT the GP tool) - does this give you any better error?
2) if above fails - try synching the changes using a Delta file instead. So this would mean first exporting the changes from the parent to a delta .gdb (or .xml) and importing on the child. Does this work? If not, does it fail on the export or the import?
If you are unable to get the above working please submit this issue to support.
For example, if 2) fails on the import, you should include the Delta File and the target .gdb to the support analyst.
If need be, you can recreate the replica (assuming the target is just a copy of the source with a one way) - but ideally we need to sort out what's up.
Thanks,
Heather