Fresh installation or upgrade of Backup Agent for Windows build 12.5.14800 fails with return value 1603. In MSI installation log, you see this error:
1: MsiManagedMachineServiceInstall:Error 0xf50009: Failed to add existing user 'MMM\NNN' to service users. | line: 0xd2c4f13c2d5bfdaa | file: e:\546\inst_supp\msi_supp\user.cpp:685 | function: AddExistingUserToServiceUsers | $module: custom_actions_msp_vsa64_s_14800 | | error 0xf50002: Failed to add user 'MMM\NNN' to group 'Backup Operators'. | line: 0xd2c4f13c2d5bfb74 | file: e:\546\inst_supp\msi_supp\user.cpp:119 | function: `anonymous-namespace'::AddUserToGroups | $module: custom_actions_msp_vsa64_s_14800 | | error 0x140bc0 | line: 0x8e9561535b69d78f | file: e:\546\core\winex\user_iterator.cpp:332 | function: Windows::LocalGroupAddMembers | $module: custom_actions_msp_vsa64_s_14800 CustomAction MsiMmsInstallInstall_MspManagedMachineService_II returned actual error code 1603 (note this may not be 100% accurate if translation happened inside sandbox)
Issue happens if the service user has the same name as the machine. (eg. the device name is "Server" and user name is "server")
The issue has been fixed in Acronis Cyber Protect 15 Update 1. If you have active maintenance, you can upgrade at no charge: see Acronis Cyber Protect 15: How to download a free upgrade.
If using the latest version is not an option, as workaround please use a service account with name, different from device name or use LocalSystem as the service account