HTTP 500 Error unable to customise

Up until recently my site has been working fine, I did update my Catch Everest Pro earlier this year. Now I get a HTTP 500 error if I try to use the customise function, however I can access the theme function option. After looking through support queries, I have deinstalled all the plugins, and also saved permalinks neither of these actions has helped. site is http://www.peterday.com.au Many thanks Sandie