Hi,
I have just upgraded my store from 1.4.7 to 1.5.17 and after upgrading I can not reach my front office, it gives internal server error. But I can log in to my admin panel. Additionally, I can not open some pages in admin panel (modules page, permissions page and some other pages..). That pages give same internal server error.. You can see a error page's screen shot in attached file.
Thank you