So … this is perhaps curious, but it is not the “OPs case”, because that message is not “challenge does not exist” - instead it is at least “a good explanation”.
I replied to the email that was sent directly to me from GreenAsJade only to find that my reply was not delivered. Therefore, I am copying that message below.
I apologize for the incorrect use of my terminology. I am not very familiar with this site and used the term “invitation” rather loosely. I realize that when I used the term “invitation,” what I meant was challenge. What I did was to go to the page where all open challenges are listed, selected Slowgoat2355(2k) listed under correspondence games as my possible opponent, and pushed the “accept” button twice, first to see the game rule and the next to complete the challenge process. Then, the response that I received was the message that reads, “Challenge does not exist.” Yet, Slowgoat2355 still remained on the open challenge list afterwards. In fact I went through this process several times today and saw the same message every time, “Challenge does not exist.” Yet, the open challenge from Slowgoat2355 is still listed in the open challenge list.
Five days ago, I responded to a request from GreenAsJade to clarify the term “Invitation,” explaining that I used the terrm incorrectly, when I meant an open challenge in the available game list. At the same time, I stated that, when I click the accept button to a game on the list, a message that reads “Challenge does not exist” appears sometimes even when the game continues to be bedisplayed on the “Available Game” list. At that time, you said, if it happens, it must be a bug. The same situation is occuring right now. When I click the accept button to a game with Cainz(2k) on the correspondenance game list, the message appears. I hope one of you would fix this bug.