Your screenshot is of 2.5.12, not 2.6 beta 5.
That aside, the database format has changed in 2.6, so when you upgrade to 2.6, once you start 2.6 and make
any change to anything in your database, it will be upgraded to the new format. If you decide then to downgrade back to 2.5.12, the software won't understand the new database format, and you'll see this error. If you really are trying to downgrade back to 2.5.12, you should restore the backup you made when you upgraded to 2.6. You did create a backup, right?