No idea, why this version hasn´t been as rigorously tested as earlier versions. There are numerous changes to the main tables that come without any warning – numerous incompatibilities even with disabled plugins and the first time that I am really in trouble with WordPress. Fortunately I was working with backups and did a pre-flight. This is what I recommend

1. go back to your last WORKING version of database and scripts
2. DISABLE all plugins
3. select the STANDARD THEME
4. maybe you need to fake the database ID in the options table?
5. delete all scripts except those in /WP-CONTENT
6. copy the new scripts to the server
7. login in as admin and wait until the system updated your database tables
8. update also all PLUGINS and activate one by one EXCEPT UltimateTagWarrior
9. import tags from UltimateTagWarrior under Admin/Manage/Import
10. the current EventCalendar will disable your site search – leave it inactive for now
11. select YOUR theme

The only thing that really helps is to cut and paste any error message into a google search: You may also want to check these links 1 + 2.
Sorry, if you see here occasionally popping up error messages – I am not sure if I really found all quirks.