In the Exchange portal, when I attempt to remove a mobile device, it generates an error message stating that the mobile device cannot be found. However, I can remove the mobile device by using a powershell session and the identity parameter of the mobile device. It appears that the Exchange portal attempts to use the device name rather than the identity when running the Remove-MobileDevice cmdlet.

For screenshots, please see: http://byronwright.blogspot.ca/2018/03/o365-removing-mobile-device-fails-in.html

Please note that this was raised with O365 support and they recommended this as a means to get the error potentially fixed.