I was not able to join any public games for the last 3 days been gettin the same error 240004.
to fix the problem all i did was , log out of the game , log out of Battle net app
then log back in fresh into batttle net app , then click on settings next to the play button and then click on Scan and repair … it will take a few seconds to check all the files and im sure it will not find any errors , however once the scans complete
click on play to luanch D3 and you should be able to join public games without any errors