This is a database corruption issue. Several people have reported it and I've been trying like crazy to identify it.
Please see this link:
[Researching] - System.Data.SQLite.SQLiteException (0x80004005): database disk image is malformed database disk imag
There is a tool there that you can download that "fixes" the problem. What it does is cleans up the bad database table. If its the items table (and it probably is), run that. Then after that, RAT should start, then do a File -> Import Items/Blocks in RAT to rewrite that data. Hopefully that fixes the issue for now.
Sorry for the trouble!