I found it. If I revert the change of Preferences class>>#doesNotUnderstand: from 8/27/215 that came with System-topa.764 it works again. I don’t know if this is the right fix as I am sure there was some good reason for the change. Tobias?

Cheers,
Bernhard

P.S.
Here is a strange thing. The mail from commits at source.squeak.org look like this:

Tobias Pape uploaded a new version of System to project The Trunk:
http://source.squeak.org/trunk/System-topa.764.mcz

==================== Summary ====================

Name: System-topa.764
Author: topa
Time: 27 August 2015, 11:47:04.664 pm
UUID: c205a736-8aff-48c5-8b3b-24ec808d19fd
Ancestors: System-topa.763

Do not return nil for unknown preferences in dNU magic but rather bail.

=============== Diff against System-topa.763 ===============

Item was changed:
 ----- Method: Preferences class>>doesNotUnderstand: (in category 'get/set') -----
 doesNotUnderstand: aMessage
 	"Interpret unary message selectors as preference id."
 	
 	^ aMessage arguments size > 0
 		ifTrue: [super doesNotUnderstand: aMessage]
+ 		ifFalse: [
+ 			self
+ 				valueOfPreference: aMessage selector
+ 				ifAbsent: [super doesNotUnderstand: aMessage]]!
- 		ifFalse: [self valueOfPreference: aMessage selector]!

However, in my trunk image the previous version of the method with the stamp sw 11/11/1998 11:40 looks like this:

doesNotUnderstand: aMessage 
	"Look up the message selector as a flag."
	aMessage arguments size > 0
		ifTrue: [^ super doesNotUnderstand: aMessage].
	^ self valueOfFlag: aMessage selector

Does anyone have an explanation why the intermediate version isn’t in my trunk image?

> Am 20.09.2015 um 15:40 schrieb Bernhard Pieber <bernhard at pieber.com>:
> 
> Dear Squeakers,
> 
> When I wanted to disable the docking bar I ran into a debugger in an image with latest update #15196. It does not happen in the Squeak5.1-15113.image from the FTP server.
> 
> The strange thing is that all the showWorldMainDockingBar(:) methods have not changed for years. So far I did not find the reason for this. Maybe someone of you have a suspicion of what change might have caused that.
> 
> Cheers,
> Bernhard