Difference in effective length between states is too big. Transition preview will be disabled

Since this error my running and idle animation aren’t working.
What does it mean and what do I have to do?

This is a bug with Unity. When you create a new transition, it’s supposed to set the Exit time to 0.75 by default. For some reason, it doesn’t in some cases.

Select your transition, if Has Exit Time is checked, set the Exit Time to 0.75 (default), or whatever you prefer. Otherwise uncheck Has Exit Time.