I can neither click into the code nor edit it. When I click "Insert", the following code is written to the editor:
Yes, I am Admin when I try to edit HTML pages and I have not been working with security permissions yet so they should be ok... I did not change any file permissions on the server, but maybe my hoster (it is a free one) set the permissions too restrictive? Would that be an idea?
On closer review it looks like all the plugins are not loading for Advcode either in the right order or at all.
Can you watch the loading in the Net tab under Firebug? I can get this to happen when gecko.js (in Firefox) is removed or delayed. This would mean a hosting or file issue from your server to your browser.