Interesting. That means it throws an error if it can't connect to X, and that it does try to connect to X, that it evidently normally succeeds in doing so (or thinks it has) and yet its window does not appear. Moreover, all this works in Mint. Of course, we don't know what that distro uses for its X server, so that doesn't necessarily mean much.
I don't expect it to make a difference, but could you just try:
export DISPLAY=:0 && ./rbp rb.im
just in case your display isn't on 0:0 but something else like 1:0.
Also, I notice the usage string incorporated a "-gui" option. What is that for, and is there, perhaps, a configuration file associated with this program which might have got lost in the upgrade?