Odd/Weird errors with FTPLib

[Bakes]

I am using a simple python script to download my logfiles. This is on
a while loop, the logfile grows rapidly, so it is necessary for python
to start downloading the new script as soon as it has finished the
old.

It works fine (for about 20 minutes), then crashes. I have removed a
couple of excepts, and have narrowed the error down to a 'error_perm:
550 logfile.log: The data is invalid.' error.

Does anyone know what the problem might be regarding this, and what I
might do to fix it?

It's really irritating to have it break every so often, and at the
moment, I have no chance in which to fix the error.