Tetra-WebBBS Support Forum
First you should check if the archived messages are actually where you expect.
Take a look at your admin entry script. Look what you've set $archive_subdir to.
Go to the folder specified and check see if the messages are sitting in that folder and if a "messagelist" file is in the folder
Then set up an admin entry script for the archive. Remember: each forum requires a public entry script for access by the public and an admin entry script for the admin(s). "Archive" is just a specialized forum but still a forum (because it has it's own datafolder).
Ensure to use an appropriate boardname for the archive. Otherwise your cookies will cause trouble
Then point your browser to the new admin entry for the archive and see what you get there. Login and all other stuff should be just the same as with your live forum.
When admin login is OK, then use the links in the bottom section of the admin portal to check database integrity for the archive forum and see what's shown there. Maybe you need to do a rebuild?
Let's see if this helps to get you going. Report errors on your way and I'll try to track 'em down with you.