The conversation with ID '%.*ls' and initiator %d references a missing conversation group '%.*ls'. Run DBCC CHECKDB to analyze and repair the database.
The metadata layer returned NULL for the conversation group. The database has been corrupted in some way. One possible source of corruption is a disk error.
Run DBCC CHECKDB in repair mode to bring the database back into a consistent state. It may delete messages if necessary to restore consistency. Investigate system error logs to see if this error was caused by another failure in the system.