Pop Up! Plugin do not work

Hello together,

i just installed the Pop Up! Plugin. I activated the add on to check if there is an issue buteverything was fine. i activated it on a specific blog not sidewide. i run a multisite network with subdomains. now i tried to add an pop over but every time i try i got the message: Pop Over not added.

Hope someone can give me a hint where i can start to search for resolving the issue. i installed the blog mu theme...