If you are having any issues logging in, or uploading files, creating directories, etc.. you can run proftpd in debug mode to see what the proftpd server is doing and possibly find out why it's generating the errors it is.
To do so, type the following:
/etc/init.d/proftpd stop
proftpd -n -d 20
Then use whatever client again to generate the error. Â Proftpd will dump out a lot of information. Â The errors you want usually happen right near the bottom of all of the output. Â Once you find what you need, press ctrl-c to quit the daemon. Â DA should automatically start proftpd up again in normal mode within a minute, or just start it with the same command as above, but with "start" instead of "stop.