550 errors and file exist filters

During a transfer, SmartFTP continually gives "550 Can't check for file existence" errors. I can see no reason why - it should be trivial to check if a file exists.

At the same time, my file exist rules seem not to be working (I have set them to only upload newer files). Instead, SmartFTP asked me the first time that it encountered a file that had the same name, and I could not tell it to only upload if newer. (Note that the fact it was able to see there was a file with the same name belies the message it keeps giving)

What needs to be done to get this important functionality working?