This error occurred on some devices following the rollout of version 18.104.22.168 in March 2020. The error cannot be resolved through the usual method to repair index files.
v22.214.171.124 was a significant change which converted the local index files from flat form to SQL and some conversions failed if the device was rebooted halfway through the process.
The index file conversion does not affect backed-up data.
There is a hotfix available which resolves this issue. Please contact us with the name of the affected account and backup set.