Add Media button not working

Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the default theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

– re-uploading all files & folders – except the wp-content folder and wp-config.php & root .htaccess files – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.

-manually updated wp-admin and wp-includes, added new files – no success
-deactivated all plugins – no success
-there are no more inactive plugins

I will now try to -reset the plugins folder
after that I will change the theme to a default theme even though it does work on my local install

indeed!
it is one of the plugins!
-renamed the plugins folder and the posts panel works as intended.
– Will now uninstall all plugins and re-install

or is there another option?

Once you have determined which plugin it was, you need to delete it or inactivate it and, if necessary, contact the plugin’s developer via that plugin’s dedicated forum via its page in the Plugin Repository.

IT seems to be the Akismet

Another installment:
Akismet or any other plugins are not at fault.

After a complete clean install of WordPress, including a new database and initially after setup everything working fine. The same problem with the Add media button came back after logging into the account from another computer.
Additionally the problem seems to get worse now, while replying to comments just worked a minute ago, after logging out and back in replying to comments does not seem to work any more either.

Can this be 1. a virus, 2. a problem with the hosting provider?

Problem fixed by moving to a different hosting provider!