I had a very strange issue here, maybe @maddenpj knows (?). 

If I tried to write a file that didn't exist, sx would complain (presumably because it didn't have the necessary file permissions). 

My end hack was to create the file from the operating system and have the server update the file with the 'r+' permission, and sx stopped complaining.

I tried various other things, but maybe I just don't 'get' python file modes. In the end, I hope this doesn't create a race condition where two people try to write/read to the same file, but this could be a future issue.