I know this was kinod of old, but since I found the problem and have a solution, figured I’d go ahead and post for other Googlers…
I am running multisite with custom content, plugin and theme directories. I too got the error mentioned above, yet the problem is in line 27 and 80 of /mp-includes/class/MP_Addons.class.php
if the PLUGINDIR is defined (as it is in my wp-config) then you will need to change the following from:
$file = ABSPATH . PLUGINDIR . "/$addon_file";
to:
$file = PLUGINDIR . "/$addon_file";
as the ABSPATH is already implied in the PLUGINDIR.
