@jeffloven @Chris_King @Jaimon_Jose @luismgarcia Yes, it is related to the new System Integrity Protection feature in 10.11. The workaround, as described in this thread Context Menu on OSX 10.11 (El Capitan) is to disable SIP.
The next version of Insync will no longer need that workaround, and we are working on making it available at least for forums testing within the week. We apologize for the inconvenience!