sorry should have elaborated more.
I was resizing the photos, so it created temp versions, sending as jpeg. And as I tried it later again, the progress dialog did not appear. Not even the resizing files progress bar (or maybe just very quickly) and I was back in the contact sheet. It's good 4.6 has the new FTP traffic lights feature, with today's speedy internet connections I might have thought the files were going out so quickly I never noticed the upload progress dialog ;-)
Mind you the cache drive was completely full by then, as I was running PS as well, and it seems PS's virtual memory is very "leaky" - when converting RAWs in CameraRaw,and saving the lot of them it seems it allocates virtual memory for each photo but doesn't release it until the end of the complete operation, which can very quickly fill a smaller cache drive which I had previously getting full without me noticing. Routinely PS fills gigabytes easily when converting few dozens of photos. So that's why the drive ran low on space. That's the famous Adobe memory allocation :-( It should release the space after each photo from the list is converted, but enough ranting...
Although, I do seem to remember that in some previous version photomechanic did show an error along the lines FTP couldn't be completed right in the middle of the resizing photos for transfer progress bar if the cache drive got full, maybe now it didn't because the drive was completely full almost right from the start. Dunno.