In a SharePoint farm with multiple servers, features have to be deployed to every server. No problem you would think. That is why we got solutions and deploy them with stsadm. SharePoint will do the rest.

But what do you do if you got an error during the deployment process?
In my case the error was:

A deployment or retraction is already under way for the solution xyz, and only one deployment or retraction at a time is supported.

I went through the ULS logs, and found this:

The job definition "Microsoft.SharePoint.Administration.SPSolutionDeploymentJobDefinition" (id "4e323369-0cc7-4cd7-ae90-850e6706df08") requires the Administration Windows Service ("SPAdmin"), but this service is not currently running.

The entry appeared on the last server I checked…
The next steps to get the solution upgraded are:

1. delete the deployment timer definition (Central Administration > Operations > Timer Job Definitions)
2. upgrade the solution with stsadm –o upgradesolution