Hi Simon,
Thanks for reaching out on this, post validation with our internal product team it was called out that we had experienced an intermittent issue yesterday night for Azure Dev Test Labs in South Central US region, however that issue was identified and a hotfix
was deployed and the issue stands to be resolved now.
Would request you to check the Azure Portal and your Azure Dev Test Labs resource now and call out if you still see any issue. Apologies for any inconvenience this might have caused.
Thank you again for using Azure Dev Test Labs.