I already found my solution and thought I would share it since I didn't find any answer anywhere. At start-up, HLK Studio crashes with an System.Windows.Markup.XamlParseException error.
It started doing that on multiple machines, and uninstall/reinstall didn't help. So I analyzed what it does with ProcMon and found that before crashing it's reading stuff in the registry.
So I renamed the key and restarted the studio and this time it worked. Solution below.