What worked for me is the following:
in
modules/mod_cblogin/mod_cblogin.php
i changed line 512 to:
[code:1]
// echo '<input type="hidden" name="return" value="' . htmlspecialchars( $login ) . '" />'."\n";
echo '<input type="hidden" name="return" value="'.$login.'" />'."\n";
[/code:1]
(the 'old' line 512 is now commented, and the new one inserted after it.)
No more login problems here.