When a post is made, does the last time update when visiting Ultimate.cgi?
This sounds like a "simple" permissions issue - please ask your web hosting provider if they recently did anything that would impact filesystem permissions on your site.
The NonCGI and Members directory should have Read, Write, and Delete permissions (or anything up to or including Full Control) for the IUSR_ account on the machine. These permissions should be set recursively, so that all files and folders within those directories also get those permissions.
Please note that 5.39 is very outdated (it's over five years old) and has numerous known bugs, flaws, and vulnerabilities. I would advise upgrading to the latest release immediately after resolving these problems.