out of curiosity what's the name of the area that the layout has been added to? we've got a possible fix for it coming if it includes spaces (encoding mixup).

Yeah some areas have spaces in their names e.g. 'footer one'. but as i renamed them to one-word the problem stays. meanwhile i have made a simple clear installation on another subdomain just to see what it should look like...and what should i say...it looks damned different! On th clean install i see thoses beautiful slidergrips and the plus-icon on the left hand. on my upgrade-test domain this isn't what i can see...no slidergrip, no edit-icon....mmmh, so what could that be?

I 'have some Jquery plugs involved...I#ll check that

if you've got a javascript error being thrown by your code, then it might make the layouts break before they load correctly. you might want to use firebug (getfirebug.com) to have a look.

otherwise, maybe the install process didn't finish correctly or something?

I've checked all my plugins and disabled everything i could in my code, but the behaviour is still the same.

The upgrade process did not throw any kind of error. It was displaying a successful upgrade, but it was fast as hell. I remember the upgrade process from 5.3.0 -> 5.3.3.1 was much more time consuming and throwed several errors...mmmmh.