Question: I have 20+ WP sites installed for clients on different web hotels. All WP-installs are updated to run latest versions of everything.

On some of my sites, the update- and install screens work as expected when I update or install a theme/plugin, the page updates and ends displaying the sucess/finished message.

On some of my sites however, these screens always just show the first line like "downloading package" (or something like that, you get the point), but then they never progress. It seems however that everything always gets installed updated as it should. Even when I do a batch update of more plugins.

It seems to be a hosting issue, as it's the same for all sites on the same host.

Does anyone have a hint at what it might be in the server configuration that is preventing these screens from updating on the progress?

Answer: 
* Deactivate all plugins
* Switch back to the default theme

… try again.

* Delete the `~/wp-content/upgrades` folder.

… try again.

* Check your `.htaccess` settings.

Things to inspect:

* What server is it? Windows/Linux/...?
* Check `phpinfo()` for differences.
* ...
I think I found a way to get it work. If it is your own server you can try to change the php-mode from Fast-CGI to ModPHP/SuPHP - Those both modes work for me in this case. I wasn't able to activate it for FastCGI tho.
If someone does know an answer to this, please let me know.