Interest can work in an economy with an inflexible monetary base (Bitcoin). In the example of a bank with 20 mil BTC offering 6%, it would soon go bankrupt, releasing BTC to the system and restoring normal conditions.
I would like to hear an explanation for how interest can work in conjunction with an inflexible money supply. Given enough time, any percentage greater than zero being offered as interest would lead to more money being promised than is able to exist. If this happens, it may not mean the whole thing collapses right then and there, but it would mean that at some point in the future, somebody would be getting screwed.
As far as the part in italics goes, I just want to know how you envision this happening.