Challenge
The Update for Veeam ONE fails during the installation with the error message "Length cannot be less than zero".
Cause
User Principal Name or UPN (user@domain) is currently unsupported with Veeam ONE. User Logon Name or ULN (domain\user) is the only supported username format for the product.
Solution
In order to solve the issue, amend the Veeam ONE Monitor and Veeam ONE Reporter service credentials prefix to Domain\Username format:
1. Open Services snap-in on the Veeam ONE server;
2. Find the Veeam ONE Monitor Server service and open the properties;
3. Navigate to the Log On tab;
4. Update the credentials prefix to Domain\Username format;
5. Restart the service to apply the changes;
6. Perform the same steps for Veeam ONE Reporter Server service;
7. Install the Update.