Assignment Selection Placement error

[ImreKinnunen_1]

When I'm trying to launch my external tool deep linking assignment selection from the Canvas assignment settings submission type->external tool menu I get "Page Error. Something broke unexpectedly."

I'm not logging anything on my server, so I think the request is not sent at all from canvas.

A submission type placement with the same developer key and same parameters as the assignment selection placement is working properly.
All I can think of is the assignment selection placement needing some additional parameters?

Oh and link selection placement with the same parameters is not working either...

Thank you for any help.