Hi Tom.
We've not seen this error before, however please check that the following services are running:
- 'SharePoint User Code Host' service on the SharePoint server (go to services.msc)
- The 'Microsoft SharePoint Foundation Sandboxed Code Service' is running (go to SharePoint Central Admin | System Settings | Manage Services on server).
After checking the above, try uploading and activating the solution again.
Otherwise, there are some other suggestions here you could try.
I hope that helps.
Kind regards,