Locked timeline

I use 18.08.01 portable on Linux. It worked very fine untill now, but suddenly the timeline has been locked.
Meanwhile I write this, I could open the padlock, I don’t know, how, but it is but it is impossible to insert a clip in the timeline. There are crosswise lines over the timeline.
Edited: The padlock is closed again, I can not open it.

Click the lock icon or press Ctrl+L to toggle it from locked to unlocked.