If it wasn't so sad it would be funny. Just got back from my last customer of the day with exactly the the same issue as you mention.
Had issues upgrading a 2014 to 2016 version so called support in Danmark. After the usual 'What's your partner number' etc (that part i can understand) THE question came at last 'Have you created a case on the protal?'. No i didn't, i have an issue now that causes the whole system to be down so i don't want to wait for a reply 'sometime' through the portal.
But, nothing could be done, they created the case for me. Got that nice litle mail to confirm uploaded some logs and then...just sit tight and wait is what is apparently expected you should do.
I have to admit, it was allready 16:45 so i supose their shift was over by the time they got the logs.
So, fixed it myself as i did on previous ocasions if you can't wait for them to reply. This is my remedy (it does supose you took a backup of the config before you started :-) as everyone does off course)
Uninstall the reluctant software
Remove all Milestone folders everywhere
Remove Milestone registry entries
Reboot the server
re-install the software from scratch
load the backup config
Try to upgrade again
Good to go
Sure, all this takes a couple of hours but it beats the support speed most of the time (probably unless you pay extra on top of CARE Plus)
I guess this just happens as companys grow and start 'streamlining their support to even better serve their customers bla bla bla'. I remember the early days when you could just call Danmark, had a guy on the phone who knew his stuff and helped you out of the mess nice and easy. That probably just means i'm getting older :-)
However, open to alternatives as well just as U1 above