How to close broken bus line?

Finally I found the cause of the problem:

There was a leftover stop in the demolished area. Somehow this stop was stuck in the middle of the forest, with no roads around, and couldn't be interacted with. I think it must be a bug in the game.

I built a road close to it (but not on it) then dragged the line to go on that road. After this I was able to remove the stray stop and the line automatically connected up its lose ends.

I hope this will help others who encounter the same problem.

I just figured out another thing that could be causing it. In my city I had changed the traffic routing such that you couldn't get from one stop to the other. Once I fixed the traffic routing the bus line self-connected

I also had this problem a bunch of times and almost always ended up deleting the whole line and creating a new one. I just finaly found a solution.

The problem was the one Carl mentioned. "Ghost" stops at deleted stations.

The solution is to select the broken line, click on a stop on the right and then click through the stops until you reach the ghost stop. That window has a "delete stop" button, wich aktivates when pressing "alt". Delet the stop and the line will connect the stop bevor that one with the one after that one. Then drag the line on a new stop, if needed.

Cheers