I just tried to use Plugin Manager from my office machine - Win7/64pro and some layers of group policy about security. And an unprivileged account.
On installing a plugin, the doxnload takes place, I get asked to allow N++ to close and restart, say yes... and nothing happens: the dialog remains open and N++ as well.
There is a plugin_install_temp\plugin1 folder with the downloaded dll and a dow9AB9.tmp file.
Now upon double clicking it open N++ prompts for closing and restarting. Choosing yes or no doesn't make any difference or action. Deleting the folder neither does. Note that launching the program from Explorer's menu does not display the message box.
Installing a plugin manually works fine, thanks.