pprune uses vBulletin 3.8.7 which is a minor upgrade from version 3.6 that it used earlier. There were certainly compatibility issues with IE8 and 3.6. Another similar forum uses version 4.1.3 and has no issues with IE9 while it runs in compatiblity mode.
With Pprune, despite it being listed in Compatibility View does not show a CV symbol on the address bar. The Compatibility View in the Tools menu is also greyed out.