Found with JustDecompile :
 [DefaultValue(-1)]
        public int PageIndex
        {
            get
            {
                return (int)base.GetValue(RadDataPager.PageIndexProperty);
            }
            set
            {
                base.SetValue(RadDataPager.PageIndexProperty, value);
            }
        }

When binding values to PageSize and PageIndex, the default value cause erratic exceptions... 

System.ArgumentOutOfRangeException: PageIndex cannot be negative when PageSize is positive.

I actually never set negative value, it seems to be caused by your default value...

I also get this sometimes :
PageIndex can only be set to -1 when the PageSize is 0

I don't see how 0 could be a valid value in this case, and I never set it to 0, Why would I want to put -1 as PageIndex? 0 would be the right default value IMHO.

How can I make it work?