It's possible that the 500 errors were something temporary with your server. 500 errors indicate there is a problem with the server serving the page. If the page didn't exist, it would be a 404 error. If the errors continue after fixing the PHP problem, then you should contact your web host to see if they can provide more insight to that issue.
But for help on the PHP error, it would be best to ask your question in the support forum for the WP Knowledge Base theme, where the developers and users familiar with that theme can help you.