Hello@HappyCookie - Thank you for reaching out to us and I apologize for the delayed response!
Are you using any other backup solution like veeam/comvault on the same Azure backup server?
If you are using any anti-virus software, ensure to add the exclusion as mentioned in the doc here - https://github.com/MicrosoftDocs/azure-docs/blob/master/articles/backup/backup-azure-mabs-troubleshoot.md#configure-antivirus-for-mabs-server
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