Are you updating from any previous version? Is it showing this error after update or all of a sudden?
Version 4.8 is stable and tested with wordpress version 4.4+ without any issue.
Can you please try again now?
Let me know.
Ok – have deleted plugin from plugins folder again and re-installed Front End PM Pro by adding a new plugin through zip upload. The plugin installed, on activation the system hung and then produced the Gateway Timeout error.