I just searched for "500 " in my access log and realized that almost all (and there's a lot of them) the 500 errors are for googlebot.
All the links seem to work when I try to access it with my browser. I can't think what could be the reason. I have no errors.txt file. I'm not an expert on reading log files so here is one sample:
... GET /index.php/category/manufacturers/garmin/ HTTP/1.1" 500 4439 "-" "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)"
In case you need it, the site is Navigadget.