I went back and took a look as the result of a different query by a different user, but also trying to use a call forward function. Now I see your issue. When I looked before, I would see the extension and it would not be grayed out and I could select it. Not being grayed out, I guess I took that as the cue that it would work.
What I now find is that I can indeed select one time, then when trying to save it, I get -
. I got it to gray out one time, like you showed, but now it is just the pop-up. I am running the latest release 17.8 released a day or so ago.
I also find that if I try and do a call forward unconditional for anytime to an external number or VM of another extension that has VM enabled, that it still rings the associated extensions rather than following the rule. I also tried changing the rules for a presence status of away and that too made no difference.
Can you confirm that as well.