Backup error: "String too long" (4235403)

Enable diagnostic logging and review the ErdAgent.log file on the DC:

1. Open the RMAD console
2. Right click the top Recovery Manager for Active Directory node and select Properties
3. Select the Logging tab
4. Check 'Use diagnostic logging
5. Click OK
6. Run a backup against one of the DCs to reproduce the error
7. Review the ErdAgent.log file. There should be an entry proceeding the "Exception: String too long" line that will show the folder or file being enumerated (for example the log entry may look like:
   EnumerateFolder(): Process '\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy7\Windows\SYSVOL\domain\Policies)

A common location where there could be very large file names or folder paths is in the ConflictandDeleted folder in SYSVOL

If the issue with the ConfilctandDeleted folder, review the following Microsoft Technet article as well as investigate if there are any DFSR issues:

https://blogs.technet.microsoft.com/askds/2008/10/06/manually-clearing-the-conflictanddeleted-folder-in-dfsr/