@Emil I tried all combinations and came to the conclusion, that encoding of the parameter part is going wrong. From my understanding the plus sign on the Denon side is a space, but %20 or space does not work.

I'm desperately trying to use Flic Buttons to control my Denon receiver. Using the URLs work great from a browser or API testing apps like RapidAPI.

Here an example:
http://192.168.xx.xx:8080/goform/formiPhoneAppPower.xml?1+PowerOn

Problem seems to be that the URL encoding of Flic Hub is not correct for the plus sign (1+PowerOn) of the parameter. I also tried escaping it myself with %2B or using \+. Tried with the SDK, too. No success. Any work around or fix?

In general, the connection is working if I use commands like:
/goform/formiPhoneAppDirect.xml?MVUP
That's because there is no plus sign