I was configuring a windows server for Veeam Backup & Replication 6 at a customer, and after some days of work I got a weird error while running some new backup jobs just created:
Error: Client error: Failed to open VDDK disk [[XXXXXX] VMNAME/VMNAME.vmdk] ( is read-only mode - [true] ) Failed to open VMDK. Logon attempt with parameters [VC/ESX: [192.168.150.70];Port: 443;Login: [****];VMX Spec: [moref=vm-116];Snapshot mor: [snapshot-161];Transports: [hotadd]] failed because of the following errors:
Trying to run some of those failing jobs manually, I saw the job was stuck while trying to copy the vmdk disk file for about 20 minutes, without any single byte transferred. Those timeouts made me loose almost two hours of tests, but at the end I managed to find out the problem.
The strange thing was to have several jobs, all configured in the past days, going on running smoothly. Since Veeam Backup is a windows-based software, my guesses went straight to the credentials used to run the backups. As a temporary step, I had configured the Veeam services to run with domain administrator credentials; asking to customer, he had changed the administrator password in the previous days.