I also can't accept challenges on the Android client, nor can I resign from games.
I'm quite honestly considering leaving Chess.com -- it's been months and this hasn't been fixed. My friends that I play with are talking about the same. Between this and the audio ads, it's been a terrible user experience.
For the past couple of releases, I have not been able to accept any challenges using the Android app. Instead, I have to log onto the chess.com website to accept the challenge.
In the Android app, I get to challenges by clicking "Play Online", selecting "Challenges". I select the challenge I want to accept, which brings up a dialog box with the options "Accept" and "Decline". I click on the "Accept" button and the dialog disappears, but the challenge remains.
Is anyone else experiencing this problem?