Is there any sort of introductory guide for PLESK out there? Some of these options I really just cannot figure out.

What especially confuses me is the various options for permissions. I have no idea what half of those mean.

Administrators
Plesk Domain User (domain)
FPSE_randomnumbers
Plesk IIS User (IUSR_domain)
Plesk IIS User (IUSR_folderuser)
Plesk IIS WP User (IWAM_plesk(default))
Plesk Domain User (folderuser)
Web Anonymous Users
Web Applications
SYSTEM

Permissions for Web Applications
Full Control
Modify
Read & Execute
List Folder Contents
Read
Write

I am totally stumped on what FPSE_randomnumbers is. There are a few of them, all with what appear to me to be random numbers. I also have no idea on what to set for folderuser or domain IIS users.

What I have at the moment are some ASP driven forums, using Access databases, in a "database" folder inside httpdocs. The database folder is password protected to prevent public access. The forums are all basically inside a "forum" folder inside httpdocs.

On two domains in PLESK, I have the same forum code. One of them works. One of them does not. The one that does not work, it does not seem to make any difference what permission settings I made. However, it doesn't help that I'm shooting blindly.

This was all working some time back. The code is all fine, the database is fine. It's the permissions causing the problem.

I am using shared hosting for this.

Anybody able to provide some insight? I don't expect "the answer" to my problems, but any knowledge I can gain is a step in the right direction.

Permissions in Plesk are Windows permissions. You might want to check out http://technet2.microsoft.com/WindowsServer/en/Library/354f4539-982a-418c-bfe7-4d5155b83f4a1033.mspx?mfr=true
Also make sure the "ownership" of the file/folder is correct. Not just the permissions.

And that the folder has permissions to run scripts.