Internet Explorer is a dependency of League of Legends. Many facets of League of Legends, including the store and login screen, are affected by your Internet Explorer settings. This is true even if you have a different default browser. For this reason, many common errors with the PVP.Net portion of League of Legends can be resolved by restoring your Internet Explorer settings to default.
Resetting Internet Explorer Settings to Default
Issues this fixes
Store loads completely black
Store reads "Session Expired"
Store gives an error when attempting to purchase RP
How to reset Internet Explorer Settings to Default