Could be a number of factors.
A burnt taste signifies coffee that has been over roasted. A very, very dark roast tastes burnt. Further roasting will produce basically charcoal.
Rubber sounds to me like old roasted coffee. Coffee has oils which are exposed during roasting and which go rancid after several days. Most coffee is weeks and months past their roast date which makes them soggy.
More on Quora thread.
