I upgraded my 1.5.1 artio version to the newest one.

As said in your upgrade tutorial I then upgraded my (paid) virtuemart plugin.

After fixing some writing rights I was able to install the new version thru the updater.

Afterwards, my joomla install didn't work anymore.
I get te following error:

Fatal error: Cannot redeclare class sefext_com_virtuemart in /usr/home/***/public_html/components/com_sef/sef_ext/com_virtuemart.php on line 1

I've now reinstalled the old virtuemart plugin.

What do I have to change now?