Hello
It was not MD5 issue.
I have just solved it.
I put additional redirect in my htaccess (I have never used it before on sites with WB CMS) and it created a mess.
It was:
RewriteEngine On
RewriteCond %{THE_REQUEST} ^.*/index.php
RewriteRule ^(.*)index.php$
http://www.mydomain.com/$1 [R=301,L]
After removing it everything is ok.
Cheers
Grawi