For anyone running into a javascript error after or during login to the manager, it seems there’s a little flaw in the /manager/frames/1.php include file.
Change line 18, 43, 113, 135 and 158:
[tt]<script language="JavaScript" src="../media/script/session.js"></script>[/tt]
to:
[tt]<script language="JavaScript" src="media/script/session.js"></script>[/tt]