A p4 check-in failed with the following error:
Submit aborted -- fix problems then use \'p4 submit -c XXXX\'.
Some file(s) could not be transferred from client.
In my case, I had an "undo-undo" changelist where I was fixing an issue with a previous submit. To fix the issue I had made changes to another file which was showing in the changelist, however, it showed the error when I tried to submit (Submit aborted -- fix problems then use 'p4 submit -c xxxx'.)
I noticed on right click in the P4V client that the file was "not checked out" (even though it was showing in the changelist as modified).
The solution : I checked it out again manually, and was then able to submit fine.