I put some more work in and verified the following combinations:

One Click Image 3.0.x: Does not contain the bug
One Click Image 3.1: Does not contain the bug (uses Pharo 2 underneath

It seems that this is a problem of Pharo 3 with Seaside 3.1

Just to be sure:
You load Seaside 3.1 into Pharo 3.0? Do you use Metacello and 
ConfigurationOfSeaside3?

If you can you post the doIt that you used?

You edit #renderContentOn: of a component using the web based tools?

I did use Pharo 3.0 and Seaside 3.1, yes. 

I copy/pasted the installation from seaside.st:

Gofer new
    url:'http://www.smalltalkhub.com/mc/Seaside/MetacelloConfigurations/main';
    package: 'ConfigurationOfSeaside3';
    load.
((Smalltalk at: #ConfigurationOfSeaside3) project version: #stable) load.

which caused an error which went away when I removed project version: #stable 
and it installed Seaside 3.1 (not 3.0) on its own cleanly.

So my doIt was in the end:

Gofer new
    url:'http://www.smalltalkhub.com/mc/Seaside/MetacelloConfigurations/main';
    package: 'ConfigurationOfSeaside3';
    load.
((Smalltalk at: #ConfigurationOfSeaside3)) load.

Yes, I then edited renderContentOn: via web and _this_ causes renderContentOn 
having methodHasNoSourceCode on the server. 

All other versions and combinations I tried and could get up and running work 
perfectly fine with editing in web AND on the server and not overriding each 
other or any problem.

I didn't want the old Pharo 2 version with Seaside 3.1 (the one click image 
uses that) so I tried making it work with 3.0. 

This is a Pharo 3 bug. Browser is broken in Pharo 3.

Browser >> #setSelector: sends #numberOfCategoryOfElement: to a 
ClassOrganization object but #numberOfCategoryOfElement: is only implemented in 
Categorizer.

Please file a bug against Pharo 3.