Output file NNN for task NNN absent

There are a lot of possible scenarios at the base of this error. I'll try to list them all.

1. Something in your system is keeping a lock on the BOINC data directory causing a write block. Check that you have excluded the data directory from being actively scanned by your anti virus and other anti malware programs.
2. Check that the data directory doesn't have the read-only attribute checked. (*)
3. While writing the output file to disk, BOINC was abruptly ended and the file was never fully written to disk.
4. The science application has crashed and needs a restart, which can only be done by restarting BOINC. In the mean time it runs through available work but never gets the chance to start writing an output file due to the application being in crashed-mode.
5. You run a BOINC wrapper application that does not copy files using the <cop_file/> attribute, thus BOINC tries to use its own version of a soft link to point to the files and fails.

(*) Note:
In Windows Vista and above the read-only option on directories may look like it's active. However, read-only is only an option on the files in the directory, a directory itself can never be read-only. Also, if any file in the directory is read-only, it'll show an X on the read-only attribute in properties.