Given X, what could have happened to make that occur?
Given that replication failed, what could have happened to make that occur?
The link could have broken between sites. The replication could have finished and thrown an erroneous failure. Maybe someone deleted the thing being replicated mid-replication. Etc...
This kind of imagination is one of the first steps to figuring out what may have actually happened. Particularly when you don't have a clear error showing exactly what happened, you're positing scenarios, then hopefully testing them.
Over time, it becomes a habit. You hear "we would like a shorter commute", and your mind immediately goes to plausible scenarios that would shorten the commute. But habits aside, don't be afraid of the internal speculation. Somewhere in your plausible scenario list is a kernel of truth.
Before you go wading in too far, think about what might have happened. Be prepared to discard it, but having a scenario (or 5 or 10) can save you time over the long run.