Azure DevOps (On-Prem) failed backup

While trying to do a backup of my Azure DevOps 2019 on Prem installation I was getting this error

[Yesterday 9:51 AM] Spike, CBB
TF401002: The SQL Server Database Engine failed to save the database backup to path \\servername\TFSBackup.  Please grant SQL service account read/write access to that folder.
[Yesterday 9:52 AM] Spike, CBB
[Error  @09:50:58.458] Backup job Failed: TF400797: Job extension had an unhandled error: System.Data.SqlClient.SqlException (0x80131904): Cannot open backup device '\\servername\TFSBackup\Tfs_Tempaad2227d-7d80-4925-aaeb-a8e8f0f82ea6.bak'. Operating system error 1331(This user can't sign in because this account is currently disabled.).
BACKUP DATABASE is terminating abnormally.
       at Microsoft.VisualStudio.Services.Configuration.SqlDatabaseBackupRestoreComponent.Execute(String sqlStatement)
       at Microsoft.VisualStudio.Services.Configuration.SqlDatabaseBackup.Backup(ISqlConnectionInfo connectionInfo)
       at Microsoft.TeamFoundation.Admin.Jobs.DatabaseBackupJob.TakeFullBackupOfSingleDatabase(IVssRequestContext requestContext, String dataSource, String databaseName, String fullFileName, ITFLogger logger)
       at Microsoft.TeamFoundation.Admin.Jobs.VerifyBackupPermissionsJobExtension.Run(IVssRequestContext requestContext, TeamFoundationJobDefinition jobDefinition, DateTime jobQueueTime, String& resultMessage)
       at Microsoft.TeamFoundation.Framework.Server.JobRunner.ExecuteJob(IVssRequestContext requestContext)
    ClientConnectionId:00000000-0000-0000-0000-000000000000
    Error Number:3201,State:1,Class:16
    <https://teams.microsoft.com/l/message/19:4337d5ee1c4f4e0783e85f502995dd2f@thread.skype/1601995914116?tenantId=0e1609b0-5e7f-4722-8611-c7f3067cebdc&amp;groupId=c4973237-b7c6-41ea-97fb-6b724182d4de&amp;parentMessageId=1601995880671&amp;teamName=Enterprise DevOps Team&amp;channelName=dotNET DevOps&amp;createdTime=1601995914116>

I am trying to find out what account is disabled.

I have done the following:

• Have give "everyone" rights over the backup folder, full rights.
• The Azure DevOps Service Account is NT AUTHORITY\NETWORK SERVICE changing to a domain account does not fix the issue
• Have tried backup to another location

I don't think any of my accounts are disabled, what account is it using to do this task?

As it turns out the account that was using was the account running SQL server that Azure DevOps database was connected to.

When I checked that account in fact it was disabled, the weird thing was SQL was still running. When I restarted the service it stopped working.

As it turns out Security team decided that the account had done something weird, so they decided to just disable the account without telling anyone.

Re-enabled the account and now I have backup again.