cloo

Problem fixed by moving to a different hosting provider!

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?

IT seems to be the Akismet

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?

-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

have the same problem as supersnapp, turned on script debugging as suggested and the media upload button worked correctly!
When I turn script debugging off the problem comes back though.

What to do?