I am using
<f:convertNumber pattern="###,##0.00" />
for numbers conversion and it has helped me through a lot of different situations.
I hope this can help you.
The problem is that I tried using <f:convertNumber> in various ways and by doing so I'm getting the error message mentioned in the original post.
Well, a somewhat ugly solution would involve changing the double to a String to properly display and, if needed, re-convert it before using it as a number.
Well, I finished up building my own converter and now I can enter values with or without separators and decimals, and the values are allways displayed with separators. It works fine.