I run the a Linux CentOS 7 running Plesk 12.
I have now figured out the issue. I today upgraded by php version as per settings on plesk.
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This caused the issue. php 5.4 is working however php 5.6 is not. So its probably either an issue with your plugin working with 5.6 or somthinf funky with Plesks php 5.6 as the highest on CentOS 7 repos is 5.4. Plesk insert there own version for CentOS.
I decided to upgrade as php 5.4 goes out of support soon.
Hope this helps ??