I cannot sign in to Charts on my iPad but can sign in on my browser and I can sign into charts on my computer without any problems. The error message I get is:
There is an error determining which application you are signing into. Return to the application and try again.
Much as I would have liked to I have never found a way to block cookies on Firefox on the iPad. As far as I can tell they are enabled. I have reluctantly allowed pop up windows but this has not resolved the issue. I still cannot log into the charts app although I can get into the browser version with no problems.