Hello and welcome,
Sign into the Support site, then click on the hamburger menu in the top right corner. The menu that drops down should show you "My Cases", click on that and you'll find it.
Let us know if you still have trouble.
Thanks!
Just a quick information, in the past that was exactly the way to find my cases. Now it seems, this is broken again. The link is missing.
Here is a workaround: To see cases go directly to this link https://www.ubisoft.com/de-de/help/cases
(thereitcally you can change "de-de" to whatever language you need it, e.g. "en-us", "en-uk" etc.; but I have only tested it with german "de-de" and with "en-us")