Usually when this occurs, you will find a unit running in place somewhere. Click this unit and give new move orders, and this should clear up the problem. Needs to be fixed before release.
End of joasoze's quote
Yes indeed there was a unit running in place. I didn't think it was the problem. Thank you! Still needs to be fixed.
It can also happen if you have an idle city.
End of NetNomad's quote
I had multiple idle citites but this was going on for a few turns already. Late game there will always be idle cities since you don't have anything interesting to build, and since they grow or double research or income on idle. I think the whole building system needs some balance and also buildings need to be more interesting and exciting to build.