When I set the external output of vMix video device to 720x405, and encode it with Adobe Flash Media Live Encoder 3.2., Adobe says:
"Odd values of video height and with are not supported. Taken nearest even values".
So it uses the nearest values 720x404. But now I do get: "Invalid or disallowed resize settings".
However, vMix does only support video size 720x405, and not 720x404...

Is there another way to let vMix use 720x404 as external output format, being able to encode with Adobe Flash Media Live Encoder?

yes indeed, it is working here as well. Thanks for the hint!

Have another question, related to video width and height, I do not quite understand.

When I change the "Output Aspect Ratio" from "widescreen" to "normal" from menu Settings -> Display, the video in vMix gets a black border on top and at the bottom, but the video itself stays in the correct ratio. But in Adobe Encoder, although I also do get the black borders, the aspect ratio is not correct anymore: a square object becomes rectangle shaped. It looks like this aspect ratio in vMix must correspond with the width x height settings but it does not change automatically. When I use e.g. 640x480 as output, the Aspect Ratio must be set to "normal" otherwise I do get a distorted video in Adobe again. Is this expected behaviour?