ERASEFILES on remote server fails for just 1 DB

Server Version 7.6.0.29 but has been having issues for many months on prior 7.x versions.

I have 8 databases including the system and 2 SSRS DBs.

Two of them are copied over the network to the exact same folder on the backup file server.

They both copy the full and tranlog backups, full to folder-1, tranlog to folder-2, both 1st-level folders on the FS. No subfolders are involved.

I have to periodically stop the SQL Backup service on the production instance (not a great plan!) in order to release the "file is being used by another process" lock on file deletion. As space is scant, I have to keep tabs on this. Having last week, as in under 10 days ago, manually deleted files that are months older than the 15-day retention (remote), I reviewed the jobs in detail today only to find that the largest database's tranlog backup jobs are "Successful (with warnings)". Those warnings are ALL concerning the kind of message shown below:

Code:

SQL Backup log file 7.6.0.29   -SQL "BACKUP LOG [MyDatabase] TO DISK = 'E:\DB_Backups\<database>\<AUTO>.sqb' WITH ERASEFILES = 2, ERASEFILES_REMOTE = 15, FILEOPTIONS = 4, CHECKSUM, DISKRETRYINTERVAL = 30, DISKRETRYCOUNT = 10, COMPRESSION = 4, COPYTO = '\\FileServer\MYSERVER_logs',  INIT, THREADCOUNT = 6, VERIFY "   -----------------------------  ERRORS AND WARNINGS -----------------------------     12/19/2013 12:01:05 PM: Deleting old backup file: E:\DB_Backups\MyDatabase\LOG_MYSERVER_MyDatabase_20131217_114600.sqb 12/19/2013 12:01:05 PM: Deleting old backup file: E:\DB_Backups\MyDatabase\LOG_MYSERVER_MyDatabase_20131217_120100.sqb 12/19/2013 12:01:05 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_051600.sqb 12/19/2013 12:01:06 PM: Warning 161: ERASEFILES error: Failed to delete file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_051600.sqb.  Error code: 32 (The process cannot access the file because it is being used by another process). 12/19/2013 12:01:06 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_053100.sqb 12/19/2013 12:01:07 PM: Warning 161: ERASEFILES error: Failed to delete file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_053100.sqb.  Error code: 32 (The process cannot access the file because it is being used by another process). 12/19/2013 12:01:07 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_054600.sqb 12/19/2013 12:01:09 PM: Warning 161: ERASEFILES error: Failed to delete file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_054600.sqb.  Error code: 32 (The process cannot access the file because it is being used by another process). 12/19/2013 12:01:09 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_060100.sqb 12/19/2013 12:01:10 PM: Warning 161: ERASEFILES error: Failed to delete file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_060100.sqb.  Error code: 32 (The process cannot access the file because it is being used by another process). 12/19/2013 12:01:10 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_113100.sqb 12/19/2013 12:01:11 PM: Warning 161: ERASEFILES error: Failed to delete file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_113100.sqb.  Error code: 32 (The process cannot access the file because it is being used by another process). 12/19/2013 12:01:11 PM: Deleting old backup file: \\FileServer\MYSERVER_logs\LOG_MYSERVER_MyDatabase_20131204_114600.sqb

Diagnosis, please.