On the page you get an error on, right click in the content area and inspect the page. In the console log, look for an error with a URL starting with /cmsctx/
and click on it. This should bring you to another page with more details. If not, you'll have to inspect your code more as the Kentico UI doesn't provide a lot of help with this. Possibly add some try{}catch{}
blocks and log events to the event log in your code.