I have this error arising on starting a project to translate an xlsx document. For clarity below, we'll call it badfile.xlśx.
I initially tried to translate it by starting a 'documents folder' project with another xlsx file (i.e. there were two xlsx files in one folder, badfile.xlxs and goodfile.xlxs). Goodfile.xlxs I was able to translate no problem, it was only at the end that I realised that badfile.xlsx was missing (having seen, but ignored the 'No source document' error).
So I tried again with just badfile.xlxs on its own and got the same error and am unable to translate it in CT.