I experienced the following problem with the "Replace document" function:
I selected "Replace document", then selected replace document again from the next dashboard-style screen (BTW, why do we have to do this?).
When I navigated to the folder containing my source document, the document for replacement was greyed out.
I therefore pressed escape to exit the replace document dialog.
I decided to investigate, and determined that CT's copy of the source document had been deleted from the CT project folder (N.B. not my original source folder).
Becuase I use an NAS to sync between computers, I was able to consult the sync log which showed that CT's copy of the source document had been deleted just a couple of minutes earlier, i.e. whilst I was attempting to replace the document.
So CT is under some, unclear circumstances erroneously deleting the source document from the CT project file. I surmise also, that this occurred at the start of the proces and is the reason why the document was greyed out.
I was able to fix the problem by copying the source document back to the CT folder and renaming it as per CT's naming convention. I was then able to successfullly replace the document.