Could not load source 'Coroutines.cs': No source available.

My coroutine suddenly stopped working - the code goes into the IEnumerator body, but returns when it hits WaitForSeconds().

The debugger said this after the return:

Could not load source 'Coroutines.cs': No source available.

There are no errors in the editor though. I've tried restarting the project, didn't help.

I'm using Unity 2018.3.0f1.