I'll be able to test this today but I'd still like to know if the developers are aware of the incompatibility with Mojave and if there's a fix for the currently deployed plugin coming. Otherwise I'll have to work out a removal process for the entire company and re-deploy the plugin via Office 365 add-ins. I would certainly prefer the bug be fixed or an official announcement from RC stating the direction they're heading is away from the pkg installer and toward the add-in method (if that's the case).
Solved - One of my clients had the same issue with the RingCentral Outlook Add-in (Error message: Please check the current timezone in Outlook regional configuration). I followed RingCentral's KB ID 11172 and realized that an important step was missing in this process. Before attempting steps 2 and 3, go to the Mac's System Preferences > Security & Privacy > General tab > and Click the Lock to make changes > and Allow from Anywhere (just for this install). After making this change, the installation and the testing of this Add-in was successful. Now, I have a happy client. I hope that this helps you out.
Using the current version of that file, I can schedule a meeting from the Schedule button from RingCentral Meetings, changing the radio button to Outlook at the bottom. Then it's hit or miss if it will crash Outlook or not when the new Meeting window pops up. This is on a few different Macs running the latest version of MacOS 10.14.5 and Office 365 16.26 as well as Ring Central apps.
Following the process of having Outlook fully open, clicking the Get Add-ins button on the primary tool bar, My Add-ins, Add a custom add-in, paste in that long URL then sign in is a lot more work.
In an ideal and best-practice world it's created as a Jamf policy and scoped to appropriate machines and ready to install from self-service. Or at an Admin level it's setup in Office 365. Sometimes it's just easier to say "Install this from their download page and you're good to go."
If this is a Microsoft thing going forward then the plugin shouldn't be listed on the downloads page as it only partially works. Also having two different Ring Central icons in a new Meeting window is going to confuse many users.