Hi Alexander,
It would be easier to answer if you post one question per thread.
About your errors.
1. Viewstate errors can be cause by crawlers that try to manipulate hidden fields in the page before postback. If I see lots of those in rapid succession I sometimes will ban the ip address. But be careful about banning ip addresses because these errors can also happen if a user is on a slow connection and they click buttons before the page is fully loaded.
2 and 3 look like problems with your search index. I would recommend rebuild the search index. I would be careful that you don't have multiple IIS sites pointing to the same mojo installation and using the same site id because you may get contention writing to the search index and it can get out of sync.
4. is caused by a missing resource key, I just added it to my copy for FolderGalleryResources.resx
5 and 6 are just 404 page not found errors, it logs them, you can generate that error by visit yoursiteroot/somepagethatdoesntexist.aspx
7 is probably crawlers poking at your captcha service
8 means you have some pages with duplicate urls in your site, it cannot load sitemapnodes for duplicate urls. There must be at least 2 pages in your site configured for the friendly url /foto2.aspx
PageSettings change is not needed as the parentPage with id -1 will have Guid.Empty for its guid anyway.
PageTree change, ok, done in my copy.
Best,
Joe