The keyboard control throws an exception, when compilation target is Windows Phone OS 7.1. For example, the sample "ArabicSMS" provided with the libraries will throw that exception before the page load event : "0x80070057, the parameter
I don't know if it makes any sense to use this library for 7.1 (Mango). But I have the feeling that it does provide some functionality not coming with Mango. As I have absolutely no knowledge of Arabic, I can't really value what is coming from the library.
The problem comes from the "key" user control. When the key has an image instead of a text (for example the return key), the FontSize is set to 0.0. Not a problem with 7.0, but with 7.1 that doesn't get through, even though there is no text at all
Using a non zero fontSize in the SIP.cs address the problem:
double fontSize = 21;