That kind of issue can be tough to troubleshoot, since I’m not familiar with the other plugins.
What version of MapPress are you using? As a first step, please upgrade to the latest version.
After you upgrade, if you’re still having the error then please check your PHP error logs (you may need to ask your hosting service how to find them). Use the contact form to send me the whole log or just the error, and I’ll see if I can spot the problem.