OK, so the session test page is failing.
This means that the problem is on the host end.
Probably, the session.save_path is not set correctly.
This is not an issue with "Custom Code" as the host claims.
The session test page is a very simple page with minimal coding.
if consists of 2 links, the first goes to:
the querystring variable test=test is used to trigger code that sets a session variable.
the second link goes to:
without the querystring variable. The second link has no effect on the session variables value. but the value is being lost.
The host will need to fix the problem on their end, show them the session test script, send them the link and ask them to test it for themselves.