Backup is the only criteria to truncate exchange log, because log truncation both at the active and the copy locations will be handled by Exchange Replication service. Please see “Log Truncation” section in Continuous Replication Deep Dive
Check info:
1. Have you followed all requirements in this article for exchange backup?
2. Does the issue happen to all SG/Database?
3. Is there any 3rd party software on the nodes?
4. Please check all files on the volume, verify if there’s any other special files other than exchange log and database files
5. Please get VSSAdmin reports with the commands below, which can provide useful info and even error messages if they exist:
Ø List Providers
Ø List ShadowStorage
Ø List Volumes
Ø List Writers
6. Please use the methods in Troubleshooting Exchange 2007 VSS Backups to troubleshoot the issue
Resources:
Exchange VSS Writers