Acronis Backup 12.5: Backup to a managed location fails with "The specified user name or password for remote resource is incorrect" when multiple machines are backed up simultaneously

Symptoms

1. You have a Storage Node with a managed vault;
2. You set up a backup plan to back up 60+ machines to this managed vault. 
3. Backup fails with the following message:

The specified user name or password for remote resource 'bsp://...' is incorrect. Check the user name and password and retype them.