Are you using PhoneGap CLI or PhoneGap Build? If the latter, the plugin must be in the config.xml -- but I suggest you do this for the CLI as well.
Uninstall the plugin and add it back with the "--save" switch, like so:
cordova plugin add ... --save
The problem appears before building a release. I'm testing with the phonegap app.
Or ist it like SMS, which is not available while testing, but working when released?
You mean you're using the PhoneGap Developer App on your mobile device? If so, it only works with certain plugins (core & a few third-party).
You'll need to create an actual build to use any plugin you want.
Strange, every other plugin is working in the developer app. And I didn't find any note, that this will not work there.
I tried a release build and sent it to my phone - it worked!
Well, "every other plugin" is probably a core Cordova plugin. The PG Dev App only has support for core plugins plus a very few third-party plugins. You can see the whole list in their config.xml file: phonegap-app-developer/config.xml at master · phonegap/phonegap-app-developer · GitHub