In reply to Carl Hancock.

Thanks for the info. My first thought was also to deactivate Quickpay and see what happens. I can see that the error warning disappears from the control panel, but the shop catalogue still isn't showing, just shows bread text *Fatal error*: Uncaught Error: Call to a member function get_id() on null in /var/www/ outdooricentrum.dk/public_html/wp-content/plugins/woocommerce/ bla bla bla...

Someone else told me, the problem was that the shop database was not updated prior to updating Woocommerce.

I was thinking it would be a good Idea to reverse to the old version of Woocommerce, which can be downloaded, but It seems that I would have to delete the 3.0 Woocommerce plugin first. That doesn't seem safe, or is it?