If you are opening an external email (*.mgs files) in Outlook while you have CRM For Outlook installed, and you open the same msg-file multiple times or open/close in short succession, you might see this error message.
We can't open "<filename>". It is possible the file is already open, or you don't have permission to open it. To Check your permission, right-click the file folder, then click Properties
The issue is due to what is called "garbage collection" in Programming... aka whenever code releases it used resources (the msg file).
Googling this issue reveals it is a very common one that also the big add-ons for Outlook also have. It is related to Add-ins for Outlook that are not their own windows processes but are embedded into the Outlook.exe process. This result in that references cleanup is not 100% up to the add-in so essentially Outlook decides when it has time to clean up memory. Without add-ins, Outlook has full control and cleans up when they know they need it so that is the reason it does not happen there but when add-ins are there Outlook needs to be careful not to clean something an add-in is using.
A lot of post on the net indicate that the issue can be reduced with some additional code (<which we added to the product back in 2016>) but ultimately it is up to Outlook when the cleanup process happen so the extra code is not bullet-proof, It can only limit the issue a bit.
One thing I've noticed as a workaround is that if it happens then switch between a few emails in the inbox. This trigger the internal cleanup and the file can be opened again.
So I'm afraid this is nothing we can get around this issue.