TypeError: null is not an object

When I export Video/Media from Animate I often receive the error message: Adobe Media Encoder failed to encode with the following exception :

TypeError: null is not an object.

I got the message when I was working with the Classroom in a Book lesson files but did not bother to deal with it because I did not need an mp4 but thought it strange that a file, provided by Adobe Press, I could not export.

On November 4 and December 7 I was able to export from animations without a problem.

Yesterday I exported an animation to mp4 and it worked too, thought the problem was solved.

Today I worked over some details in the animation from yesterday, saved it with a new name, wanted to export it to mp4 and got the error message. 

Testing the animation from the day before I was able to export, today I could not export it anymore.

I noticed that there is an update for Media Encoder, updated, made a restart, it did not help.

Tested an animation from an Adobe Press lesson file, also get the error message.

Mac Version 12.6.1, Adobe Animate 23.0.0, Media Encoder 23.1

This does not make sense to me, appreciate if anyone has an idea what to do about it. Thank you.