When I created a 6th Customer under Management -> Customer then switched to ITSM to create some users and enroll devices, I found that I’m not able to select the 6th (or 7th) company in the Create New User dialog. Is anyone else running into the problem? How are you dealing with it?
If your problem still continues, I am just guessing it might be related with browser, but you might try to open ITSM in a new tab with right click, open in new tab actions as a workaround.
Then please try to add the same customer on new tab.