Cannot activate FTPeek on FMS

[Gnurps]

On two Filemaker Server machines I manage, I cannot get FTPeek to activate. After checking the checkbox, a grey message appears appears stating that the plugin was enabled, but the checkbox is unchecked. 

I have checked the permissions of the plugins. Both owned by fmserver and group fmsadmin. I have restarted the machines. I have tried Wim's suggestion of installing with server-side script. Both have Java 1.6 installed. Both have the BaseElements plugin installed and successfully enabled.

Both are running FMS 14.04. One is a Mac mini running Yosemite, with  Core 2 Duo processor and 8gb. Other is a Mac mini running El Capitan with Core 2 Duo and 4gb.