After BlueBerry.clean and the removal of all local config directories, the Workbench crashes on the first start from a build tree since a few Qt plugins are found/resolved in the system directories instead of the installed Qt version that was used for the build. Amazingly, just starting again works!
This really is an issue on Ubuntu at the moment. Works with installed version of the Workbench, but requires the system-version of Qt5PrintSupport to be installed with the built version of the Workbench. Then, the message mentioned above shows up and every start afterwards works.