Only a single user can login at any one time.

Explanation

This issue is probably caused because you are trying to run Numbers Up! Volcanic Panic across a network – if you try to install a single copy onto your network server, with each client computer running it over the network, you will cause an error because files cannot be opened more than once at the same time.

Solution

You may be able to install and use the program to work with a network setup, storing user data in a central location. See this post.