Can't change directory to dir name

This is an error you will see when BOINC is unable to change the slot directory's name prior to an application's start. BOINC doesn't run tasks in the data directory, but in slot directories in the data directory. Each slot directory is made at the start of the task, and removed once the task has been calculated, uploaded and reported.

Possibly your account does not have permission to do writes on the data directory. Try to reinstall BOINC to set the permission right.
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