@Jiwa, thanks for the creds.
I had a look and found that 'session' variables are not working for your site.
This could lead to several problems - the one you reported for mosaic being one of them.
Common reason for session not working are the tmp folder used by your web-server being full or it having the wrong permissions.
The resolution would be to contact your web host letting him know about the issue and asking him to make session work again.
That said, it wasn't really essential for mosaic to use the session variables so I decided to make some changes and make it work without them (this removes one potential point of failure). I have committed the changes to GitHub. Your installation is already working now so you don't necessarily have to apply the fix.
In closing, please let me reiterate - this particular issue is fixed for your setup but there could be other issues stemming from session not working (e.g. cart will definitely not work - as would flash variables). So, you'll still need to contact your provider.
Hope it helps.