All prior bugs for this issue have been fixed. That said there is still a limit to how many actionable messages you can open before you see this message.
In Current Channel its now 10 actionable messages
In Monthly Enterprise Channel its 5 actionable messages
In SAC and later channels its 5 actionable messages
The limit of 10 will go to MEC soon and progress to SAC shortly thereafter.
In the meantime, if you get that message, close any prior Actionable Messages you have open to avoid seeing the error dialog. Long term Outlook will keep iterating to make the number higher as performance allows e.g. higher than 10.
Here is the Known Issue article we have explaining the current status, Error when opening Actionable Messages - Microsoft Support.