If you’re seeing “tag is unavailable” errors when attempting to protect a VM using vSphere tags or “QueryAllCategoryInfos failed. There was no endpoint listening at https://vcenteraddress/lookupservice/sdk that could accept the message” error when editing the backup job.

I recommended investigating DNS in your environment, in my case performing a nslookup of the Center FQDN from the VBR server would fail.

Once the DNS issue was resolved (restarted the DNS server), Veeam could look up the vSphere tags without issues and the backups resumed without any further errors.