Since the latest version of Office 365 there is an error shown when you open a second Outlook instance.
"Sorry we have trouble starting Outlook. Only one version of Outlook can run at a time. Check to see if another version of Outlook is running or try restarting your computer."
´This problem only occurs with the latest version 1806 (10228.20080)
It's easy to reproduce:
Start Outlook once
In the taskbar right click the icon of the running Outlook instance and try to start a second instance.
Then this error occurs, after clicking the "Ok" button the second instance starts.
This build of Outlook has so many annoying bugs (this one, Calendar permission, Skype for Business Addin German template)
This is a known issue that has been reported. The cause is the change to a five digit build number which broke a function that assumed four digit build numbers. I have no ETA on the fix, but it's coming.
Was there a resolution to this? The issued just started on my computer. @Hal Hostetler
When you click on the Outlook icon to launch Outlook, you are presented with the above error, and Outlook fails to open.
Office 365 migration expert