Fixing Veeam 'Another task is already in progress' Error

I came across an odd issue for the first time in my lab. A Veeam backup failed to complete with one VM reporting the following error:

Error: Another task is already in progress

With retries producing the same warning, it was time for more meaningful debugging. Upon logging into vCenter, I could already see stacks of recent tasks failing with the same error. I found the VM in question, then tried to put the host it resided on into maintenance mode, forcing a compute vMotion. All VMs migrated successfully except the problem child. Time for a quick fix—I’ll just restart the guest OS.

Nope.

A bit more digging revealed that in certain cases, tasks may timeout in vCenter, continue to run on the host but fail to complete. The fix is a simple restart of management services.

SSH to the host with problem VM(s), then run the following:

/etc/init.d/hostd restart && /etc/init.d/vpxa restart

No downtime, no host restarts, no problem.