@AndyE3D @chrishamm thanks for the investigations and the fixes.
Did a quick test, stayed with 3.6.0-beta2 and updated dsy-python to 3.6.0-b2.post3:
-> No exception/exit anymore
-> String-values are correctly set via set_plugin_data
-> Object model plugin variables disappear (only last one modified is visible), accessing them via 'echo ..." works
Reloading the OM-plugin page via browser reload also brings back all values.
Will test tomorrow if using non-string values also works for me and also test 3.5.4 again.
Thanks for your help, have a good evening!