Remote harden of transaction '...' (ID ...) started at ... in database '' at LSN (...) failed.
On SQL Server 2016, now even on two versions, the same error message can be seen while AG failovers either fail, or the after failover secondary replica has their databases in REVERTING / IN RECOVERY state.
Example of these error messages
Remote harden of transaction 'IFTSAutoNested' (ID 0x00000000141b98bf 0000:1d20f419) started at Feb 19 2019 11:16AM in database 'DB4' at LSN (93341:183352:3) failed.
Remote harden of transaction 'INSERT' (ID 0x00000000141b989c 0000:1d20f418) started at Feb 19 2019 11:16AM in database 'DB4' at LSN (93341:183346:17) failed.
Two of such occurences have now appeared on dba.stackexhange.
Randi Vertongen commented
We found issues with the automatic seeding process failing, and are going to patch to SP2 CU5 as soon as possible to get it hopefully fixed with this bug fix: https://support.microsoft.com/en-us/help/4457953/access-violations-and-unhandled-exceptions-occur-in-sql-server