248 is an optional update. The new features are probably very nice, but because I cannot use v248
at all, that doesn't really matter.
And because I am also unable to easily back out the failed upgrade, my weekend stands a good chance of being ruined. A word of warning about that risk seems appropriate.
UPDATE: With a combination of reinstalling, restoring from a prior backup, and re-syncing after that ... I am now able to access my main Realm again in v245.