how did you update? did you just re-uploaded all files from v1.7.9 package and replaced the old ones? If you did, then try upload them again (and not only files in admin/ folder), and make sure all .php / .html files being uploaded in ASCII mode, not binary (warning: you will need reinstall any mods you've previously installed)
P.S.
usually server logs have more information about 500 errors, ask your provider, it might pinpoint the problem.