The first is irrelevant. And you seem to understand the second, as you mentioned that you have enough storage space.
Anyway, I have no more clues. If you find out a reason, let us know.

I'm getting the error 4 while trying to copy files from my desktop to the server.
i read the documentation - i've enough storage space and tried to rename the file name to an existing one. it didn't work out.
as for the other two options:
# Moving remote file to different filesystem (HDD).
# Uploading file to full filesystem (HDD).
i didn't quite understand their meaning.

you'll have to excuse me, i'm doing my first steps with it. :)