I don't have a lot of experience with CMO but do you have the roles CMO User and CMO Admin configured in the admin mode.
A couple of things I have seen
* CMO could not handle Swedish letter ö in a security group name
* wrong name in database after upgrade
* forgot to run the command that move all old addons to protected modules
I have a serious problem trying to get CMO working. I have all the newest CMS packages etc. the project is completely NuGet-based. I can't find any descriptions of how CMO resources should be placed in this new environment. Tried to copy all necessary settings from another CMS 7.5 site with working CMO does not help. Logging says CMO modules &c are found and registered, but the UI does not get visible. Trying /epiuipath/CMO brings to logon screen even when logged in, and is stuck there.
Any hints (cmo path authorizations have been double checked..) ?