Actually, I have managed to resolve this issue, so in case anyone’s interested, you just need to duplicate the plugin file (original file is teams, you can go for teamsv2) and also edit the header line “# Notify via Microsoft Teams” to something like “# Notify via Microsoft Teamsv2” so you can distinguish which is which from the Checkmk WATO notifications config. page.
The outcome will be two plugins, with different names, different labels but identical functionality. I just tested them and they can work in parallel. I think the issue was having the same plugin name, so Checkmk would confuse which one to call.
