As of May 11th, this issue may be resolved by updating to Windows Defender 1.1.18100.6 and clearing existing logs.
Issue: File-level backups hang on "Updating file catalog..." for an extended period of time.
This appears related to historical scan logging in Windows Defender starting with the Windows Defender engine1.1.18100.5.
Example: A 2019 server had in excess of 21 million files located in 'C:/ProgramData/Microsoft/Windows Defender/Scans/History.' Backups containing this data may take an excessive amount of time to complete.
Check for and update any Windows Defender installations as Microsoft may release an update to reduce this behavior.
General recommendation currently is to exclude the folder path: 'C:/ProgramData/Microsoft/Windows Defender/Scans/History' and run a new full file-level backup.
White it may be possible to use utilities like disk clean-up to clear out the logging, until fixed by Microsoft, expectations would be for the logs to fill back up over time leading to disk space issues and/or the concerns noted within this KB.
Unitrends is currently evaluating options with our file-level agent to help mitigate the impacts on file-level backups when this is encountered.