I posted on another forum that some mobile devices cannot check out using the combination of PayPal Advanced, X-Cart 4.6.3, and reBOOT.
http://forum.x-cart.com/showthread.php?t=69688
There is a video on youtube that shows the problem using an iPhone 4 and a Samsung Galaxy S3.
https://www.youtube.com/watch?v=XjS3cUn1ptU
PayPal support does not think the problem originates with them. Please see the other posting and let me know if you have insight on how to approach this problem.
It is a bummer that I got reBOOT because it would be mobile friendly and now some mobile customers cannot check out.