When we click on the username from the poslist, then the "link back" doesn't work on the profile page.
Is it supposed to be like that or it is a bug?

Bug. Line 222 of showprofile.php looks like this:

$formlinkstart = "<a href="{$config['phpurl']}/$what.php?Cat=$Cat">";

Change that, to this:

$forumlinkstart = "<a href="{$config['phpurl']}/$what.php?Cat=$Cat">";