Hello again Jan,
I've been able to recreate this problem in the default CorporateSite installation, and I think I've partially found what the problem is - it has something to do with changing the toolbar set for an Editable Text webpart on a pane of the Accordian Layout webpart.
To recreate the issue in my default installation of the CorporateSite, this is what I did:
I copied the "Corporate Site - Development model (Mixed)" template as a new template, and then set the Widget zone type for the Left zone to be "None".
On the Design tab of the new template, I then added an AccordianLayout panel with 2 panes, and added an EditableText webpart to each, leaving all the settings as default.
Created a new page using this new template. On the Page tab, navigating to that page didn't record any Warnings in the Event Log.
I went back to the Design tab for the template, and modified the Editable Text webpart in one of the accordian panes to use the "Full" HTML editor toolbar set.
Select the Page tab for the page, went to the Event Log, and WHALLA! There is the same error!
Event URL: /CMSPages/GetCSS.aspx?stylesheetname=0
URL referrer: http://kentico9/cms/getdoc/67f1fc85-7dfd-46c0-931a-bd80ce17ea25/Mixed-Test.aspx?viewmode=3&showpanel=1&cmscontentchanged=false&lang=en-US&langobjectlifetime=request
The guid "67f1fc#######" is the Node GUID for the relevant page.
I've re-set the HTML editor toolbar set for the Editable Text webpart in the pane back to the default "(default)" setting, cleared the browser cache, cleared the cache in the System application and re-started the application, but I'm getting that Warning recorded each and every time I navigate to that page in the Pages application if I'm on the Page tab.