I got the final solution now... If you go to the site and click "Top Teams" for example and you get the message...
"The project server is currently offline. Please try again later."
then the server is really offline and we can't upload WU's or do anything else.

But if you go there and everything works, and the top teams tables show for example, then shut down and restart boinc and everything will be fine.

The site is giving me the message now, and I have a few WU's needing to upload. (but failing to do so...) As soon as I see that the site/server is up again I'll do this procedure and I'm pretty sure it will work like yesterday.