@stormi I will add some details here:
This was my "hardcoded" value, that is not intended to be used here.
Instead the official way is to extend some Enums with supported API Versions.
If the Version of the host (for 8.3 it is 2.20) is not found in the enums, it get's set to 99 = Unknown.
The current enum contain only up to 2.16 (API_2_16) so API_2_20 is not found --> set to 99:
Problem is here where it compares the min version to the current and if it is higher (and it is with 99) it refuses to connect.
As I hardcoded it to a lower value (20) it is now lower and allows to connect.
And btw: my build was based on a slightly newer commit from Citrix than the one at xcp-ng repo.