So I started up creation kit, loaded my mod, went to the generate archive option and created a .bsa and .bsl file (yes, I manually dragged and dropped everything). I then copied these two files into my Data directory and selected upload mod to steam.
As usual, it told me that an archive already existed and it asked me if I wanted to use the existing archive. I said yes. (This is normally what I do to keep my nexus and steam versions synchronized).
So... After hitting yes, I got a modal dialogue box with the error message "Failed to enumerate published files! Steam error 16". When I hit Ok, it closed the upload window. And I appeared to be stuck. Furthermore, after this happened, every time I ran Skyrim, I got an error message "Failed to enumerate subscribed files!".
The solution... (drum roll please...)... I had to exit steam, reboot my machine and then start creation kit as the FIRST THING I did after the reboot. As long as Creation Kit is started before Skyrim, it works. But, if you start skyrim before attempting to upload the archive, then you get the error above.
I am not speculating on the possible reason for the issue. Just telling you what the fix was in case someone else runs into this.
Happy modding...