Permission error when installing or updating WPMU plugins

we do experience problems with installing/updating WPMU plugins via HUB or WP admin panel, error says that there is problem with file/folder permissions
"Installation of WP Smush Pro failed. Most likely reason for this are wrong folder permissions of your wp-contents folder."

We have tried it on a different servers and the same problem appears.

All of the folders:
have 755 permissions

We can install and update other plugins via WP admin panel. The problem seems to be linked only with the WPMU Plugins.