I have tried to use the Message+ app on OS X Mojave with no luck. The app crashes if you choose File|About. When I enable my account, any message I try to send just gets stuck on "sending" and never goes out nor to incoming messages show up in the app. I have tried removing references to the app in KeyChain Access and re-installing several times. I know everything else is correct (except this OS/Client combination) because the app works fine on my phones and on my Windows PC. So, it does seem to be related to OS X. Any message I try to send just looks like this and the status never changes.