All hosts are working fine with one exception:
When I rescan that host, the red X disappears and I can see all VMs on the host. When I start a manual backup, the red X shows up and it gives the following error:
Processing finished with errors at 3/28/2018 9:48:14 AM Unable to allocate processing resources. Error: On-host proxy [Hyper1] requires upgrade before it can be used.
All hosts are installed the same way. Checked auth/Kerberos settings and AD delegations between Hosts (not like it should have to do with this error...).
[28.03.2018 12:36:42] <01> Error Failed to dispatch pending task.
Other hosts where Backups are working there is no "failed to dispatch pending task" error.
Deleted and re-added the Host. No change.