Can't copy file to directory

You will get this error when BOINC cannot copy the files it needs to do the task calculations, to the slot directory that the calculations are going to be done in. Check that your account has permissions to do writes to the data directory and anything in it.
Else reinstall BOINC to reset the permissions on the data directory.

In the case of Linux, and when e.g. your data directory lives at var/lib/boinc, try to fix the permission on the data directory by doing chown -R boinc:boinc /var/lib/boinc