On wordpress MU, if the plugin is activated on SOME SITES and not on globally on all, it will resolve in :
1 - an error on admin pages (widgets.php and sometimes index.php) :
Fatal error: Call to undefined function dynamic_sidebar() in /path/to/wp/wp-admin/includes/widgets.php on line 80
2 - Error on Front-side :
error: Call to undefined function dynamic_sidebar() in /path/to/wp/wp-content/themes/nameoftheme/sidebar.php on line XX
This is essentially the same error (missing function dynamic_sidebar() ) that probably occurs because the plugin at some point is requesting this function too early ?
I can confirm that this happens on ALL themes installations (also wp default ones, and hence not a theme problem).
The sites where this plugin was activated will not have any problems, where the sites it was NOT activated will result with the above mentioned errors.
After activating the plugin as default for all sites this problem is fixed , but it is only a temp solution because there should be an option, like with most plugins, to activate on a site-to-site basis.