crash aften upgrade to Joomla 4

what do you mean??
should I re-establish the site with Joomla 3.11???

I followed your instructions excately - as you can see from the plugins I deactivated.
And it was your plugins that made the trouble. I worked fine until I activated your plugins as mentioned above

I suspect that is exactly what she means. You should probably recover back to your Joomla 3.11 and then start the process over but this time, as you follow the instructions they have provided. Make sure to not activate multiple components, plugins, etc. at once. This way you can test at each step of the way. And, make multiple backups as you progress. This should help you to narrow in on what is causing the crash. Also, make sure that you do not install any older versions of software but instead get the latest version for each component and plugin.

I told you. I did excately that!!!
and it was system sp pagebuilder, that made it crash, when I reactivated it
everything worked fine until that
you are not helping!

OK. Sorry for not being helpful. But, let me explain why I replied the way I did.

You said

*but when I tried activating the 3rd (I think it was System sp Cookie content) it crashed - and I cannot use backend

This implied that you believed but were not sure about which plugin caused the problem.

I was interested in their reply as well. I did notice something in the document they told us to follow points to a video about how to resolve the error message that your site is presently displaying: Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()

Joomla 4 Tips - Fix Upgrade bug
https://www.youtube.com/watch?v=ZO9nmdsLlFw

sorry - if I wrote wrong
it was the third from the bottom - system sp pagebuilder
system sp pagebuilder
system sp pagebuilder pro update
system joomlaschack sys.

until that it worked fine