Not able to drop anything on Timeline

Hello

I'm new to Premiere, but for a week now I have been editing with no problem, until suddenly I could no longer drop anything on the timeline. The hand is shown, but with a crossed circle, so no way to move content (audio or video) to the timeline. I try everything suggested on the forum (selecting the patching, clearing the in and out point, etc) but nothing is working. My only solution so far is to create a new empty project, import anything, drop it on the timeline (this time it works) and then opening my working project, which, for mysterious reasons, starts working.

I'm using the latest update 12.0.1 on a MacBook Pro with El Capital 10.11.6

Any ideas how to fix it?

thanks!