That’s generally not a MapPress error, it’s usually caused by another plugin or a corrupted file.
If you know how to check your PHP error log, you should be able to find the offending file and line number there.
Another approach that usually works is to just disable ALL of your other plugins. If that fixes the problem then reactivate until you find the culprit.
In some rare cases I’ve seen this problem from a corrupted WP install, so you can also try re-downloading the WordPress files.