Rule 1: always check if C: is 100% full. Rule 2: windows shortcuts are not *nix links. Rule 3: monitoring!!

A customer of mines Veeam Backup job started failing with whis error:

Processing VMNAME Error: Failed to open VDDK disk [[LUN14] vmname/vmname_2.vmdk] ( is read-only mode - [true] )
Failed to create folder. Folder path: [C:\Windows\TEMP\VeeamBackup\VeeamAgent-9500\5eea325f.533am].
Logon attempt with parameters [VC/ESX: [vCenter.lan];Port: 443;Login: [veeam_backup];VMX Spec: [moref=vm-408];Snapshot mor: [snapshot-679];Transports: [san];Read Only: [true]] failed because of the following errors:
Failed to create folder. Folder path: [C:\Windows\TEMP\VeeamBackup\VeeamAgent-9500\81ecb8a1.533am].        

After some troubleshooting process, I understood one of the proxy had problems. I disabled it and all jobs were running fine again.

After some more troubleshooting on the operating system of the proxy (event viewer and so on) I discovered the easiest: C: was at 100%!!!

Rule 1: if you have some problem on a machine, check disk space first!

The C:\ProgramData\Veeam\Backup directory was more than 40GBytes

While waiting for suggestions from the Veeam support, I decided to try if the windows shortcuts work the same way as symbolic links in linux. I moved the big directory to drive D: and then created a shortcut to replace it on disk C:

After restarting the services, I found in C:\ProgramData\Veeam two Backups: the shortcut and a newly created directory :-D

Rule 2: windows shortcuts are not *nix links

The solution was easy: check https://www.veeam.com/kb1825

This registry value is not present by default and may need to be manually created.
Determines where log files created by Veeam Backup & Replication will be stored.

Key Location: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication
Value Name: LogDirectory
Value Type: String Value
Value Data: <path_on_server>
Default: C:\ProgramData\Veeam\Backup        

this way you can redirect the heavy (in big environments) logs to another local drive.

(the content of C:\ProgramData\Veeam\Backup is just logging, you can safely delete them)

Lesson to learn:

Rule 3: always apply efficent infrastructure monitoring

In example, if you are Veeam customer you probably have a valid license for Veeam ONE. If not, you can use the community version!

personally, I will always love XYMON https://xymon.sourceforge.io/