It is possible to have "{4} | Page {0} of {1}, items {2} to {3} of {5}" (all 6 parameters) for PagerTextFormat, but not just "items {2} to {3} of {5}".
The PagerTextFormat is passes as a first parameter of the string.Format() function, so it must contains all six parameters into it. Otherwise the exception will be thrown. To achieve the desired functionality you can use following string as a PagerTextFormat: PagerTextFormat="{4} items {2} to {3} of {5} <span style='display:none;'> {0}{1}</span>"