Thanks to Jason's help this issue could be solved.
It seems it was not a problem with DataBridge at all but for a reason I don't quite understand, the DW folder "_notes" in the main directory could not be updated and had to be deleted entirely on the testing server (might have been an issue with access rights)
After we deleted this folder it could be re-created and the problem dissapeared, i.e. session variables could be created and showed up in the Bindings Panel.
Thanks again for a great online support, even if it was connected with DW and not with the extension.