I am having the same problem.
The Admin link at the foot of the Admin Login Page is in form:
http://domain/forum/adm/index.php?sid=e ... d0ea652aea
But the Proceed to the ACP is in the form:
http://domain/index.php?sid=8eb7bf15221 ... 1241ee8cc7
The default if one clicks neither is the same as the Proceed to the ACP - ie wrong.
Clearly the "Proceed to the ACP " version is missing the root directory of the forum; in my example = forum and the "adm" subdrectory.
It's a bug.
Tried lookking in functions.php where the redirect is set up but cannot see what is wrong.