It is looking like all Mac third party extensions need to be updated for v19.3.1 for macOS.
Although I’ve not been able to find any documentation referencing this as yet, the only plugin we use that has successfully launched is Dacons MailIt 18.104.22.168 that has recently been released and is available for download from their website.
Dacons had alerted us that there is a new SDK for plugins with this version and their plugin had to be updated. We also use 360Works Scribe and 2empowerFM, both of which are in the extensions folder, but have not loaded.
The original plugins have launched in the Windows version of v19.3.1 and, as yet to be tested fully, have all registered.
An annoying Windows only bug that we reported to Claris during the v19 pre-release version, and subsequently again last year has still not been fixed. This occurs when ‘Filter Databases’ is enabled on the server to allow users to only view the databases they have access to. Selecting a favourite server from within File -> Hosts prompts for a password and subsequently displays the database files, but upon double clicking a file, instead of the second password prompt it does nothing. ‘Show Hosts’ and then selecting the servers from within is the only way to open files. A small, but very annoying bug when accessing multiple servers.