Sebastian - I'm afraid that 'changing permissions' is not the magic fix it all solution for some problems. I don't want to risk opening up the 'security permissions' on my server, when it works locally but not live and am very hesitant to even suggest others do this without having actually tested what people are writing about, which makes offering suggestions so hard at times.
I have about 300 portals running on a mix of DNN running on 1.1, 2.04, 2.1.2, 3.1.1, 3.2.2, 3.3.4, 3.3.7, 4.3.4, 4.3.7, 4.4.1 (but upgrading the ones which have security flaws in them at the moment) And I can tell you that I know how to get these sites working and that problem is a real one.
I can confirm this is a bug. It occurs for me on 4.4.1 as well. I found the error but it required a recompile to fix up the bug in the FCKEditor. What I can't confirm at the moment is if it occured in an upgrade or a new build. It has something to do with the application path.
How you can identify there is a problem is when working with a virtual directory environment often found in the local host instance - eg. domain.com/dnn/ and then moving to a domain based one.eg. domain.com - there is inconsistent behaviour in the application path which, when I click to upload an image using FCK - it comes up with the with the full website page in the browse window with an error - something being 'null' I can't remember.
I had it fixed along with lots of other annying bugs on a special DNN build, and it also has to do with the other post in this forum - when you go to upload an image there is no button to click because it's looking to upload to a non existent folder. It's calling the location path based on the URL, not the application, hence .. something in the FCKEditor has a blank path.. which forced the need to recompile.
I intially thought it was due to my customisation of DNN but because I could identify the problem to be the same as what is here, I am confident that it's an FCKEditor issue not user related.
Perhaps some testing is needed by others to confirm - It may be only intermittent due to installs and behaviour between upgrades. But at least I can offer you some comfort in knowing that it's not 'user related' based on my own testing.
Nina Meiers