On the magritte and seaside lists Bruce Prior explains that loading stable versions of Seaside and Magritte doesn't result in having a working combination.
project: 'Magritte3' with: [
spec
className: 'ConfigurationOfMagritte3';
versionString: #'stable';
loads: #( 'Seaside' );
repository:
'http://smalltalkhub.com/mc/Magritte/Magritte3/main' ].
Stephan
