Ucs-sso only works on /univention/management

I just upgraded to ucs 5.
When I now type the url https://master.example.com
I get redirected to /univention/portal/#/ which seems right. Clicking on signin I get redirected to
/univention/login/?location=/univention/portal/ and not to https://ucs-sso.example.com.
Whereas if I type the url
https://master.example.com/univention/management and then click login the rediret co ucs-sso works

And idea?

Still struggling here. The described workaround still works, but I’m not sure how long this will last. I’m a bit afraid the next upgrade will break the workaround.