my right sidebar is frozen.
The content it contains now is not the content I put in.

The pulldown of my right sidebar in wp-admin/widgets.php seems to be empty.
Hope this sounds like something, someone recognizes. for I have no clue how to solve

The pulldown of my right sidebar in wp-admin/widgets.php

are you saying that the file 'widgets.php' in th 'wp-admin' folder is blank?

did this just happen?
what changes have you recently made?

Yes, the whole right sidebar was empty, and not by hand, so I do not know what went wrong.
Somehow It is not frozen anymore and I am able to enter widgets. However, some of them(made in a text widget are gone

Let me ask my questions again
1) you said that we-admin/widgets.php seems to be empty. Widgets.php is a program that contains code. You can not see this if you are in the backend of Wordpress, you would have to actually edit the file.
- did you edit the file and see it was empty?

2) what changes were done to the site between it last working and your discovery that the widgets were missing? For example, did you upgrade Wordpress or atahualpa or add in a new plugin or update any existing plugins?

The right sidebar looked empty (no widgets) and was not accessible at all.
I did edit the file and see it was empty.

Frontend contained widgets I did not put there and the right sidebar seemed to inhabit widgets of the nonactive left sidebar.
Since yesterday evening my right sidebar is active and accessible again, but do not know what caused it nighter what helped me get it started again.
The only changes made where updates and upgrades of WP and its plugins.

Is it still save to use atahualpa since it is no longer updated for some year now?

1) is Atahualpa safe to use - ASFAIK yes, there is nothing in any new WP upgrade that has caused it problems, if there is and I can fix it, I'll create a patch. The reason it doesn't show on WP naymore is because they theme review team made a decision two years ago, that all themes MUST use the customizer. It would take a emense amount of time to do that for atahialpa and there is no payback for the effort. (i.e. If I did it and it took me 100-200 hours to do, what would be my benifit?

2) It you truly edited wp-admin/widgets.php and it was empty, I would be very concerned. The three ways I can surmise this could happen are
1 - you didn't actually open that file, you had a typo and opened a new file tattt was empty
2 - an update of wordpress timed out or failled and the file was not created
3 - you may have been hacked.

Had you said you had just updated Atahualpa to version 3.7.24, I would have pointed you to this thread from back in May of 2015