Update:
- I've checked all permissions, etc. Using ubb6_test.cgi, it informs me that everything is OK.
- I've even tried upgrading via the full zip file for 6.05, and still getting the exact same error.
- I've nuked everything in the main cgi dir, and updated all files from the full 6.05 zip (cgi and var files). Also nuked everything in the main dir and template dir of my non-cgi dir and re-uploaded and set permissions as needed from the full 6.05 zip. Still getting the exact same error.
Dang - I know the problem is going to be something incredibly small and minor, and I'm going to kick myself very hard once I find out what it is...
