Unicode problem on GET '/api/templates/main_menu/' with german translation

When switching to the German locale I get the following non-fatal (as mayan seems to work and show pages) errors in my log.

Server is a freshly updated installtion 3.0.2 as "advanced deployment" installation on an up to date debian/stretch and a postgresql-db.

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2699: ordinal not in range(128)
common.middleware.error_logging <1665> [ERROR] "process_exception() line 17 Exception caught by request middleware; <WSGIRequest: GET '/api/templates/main_menu/'>, 'ascii' codec can't decode byte 0xc3 in position 2699: ordinal not in range(128)"
Traceback (most recent call last):
  File "/opt/mayan-edms/lib/python2.7/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/mayan-edms/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/opt/mayan-edms/lib/python2.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/views.py", line 494, in dispatch
    response = self.handle_exception(exc)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/views.py", line 454, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/views.py", line 491, in dispatch
    response = handler(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/generics.py", line 210, in get
    return self.retrieve(request, *args, **kwargs)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/mixins.py", line 58, in retrieve
    return Response(serializer.data)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/serializers.py", line 537, in data
    ret = super(Serializer, self).data
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/serializers.py", line 262, in data
    self._data = self.to_representation(self.instance)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/serializers.py", line 504, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/opt/mayan-edms/lib/python2.7/site-packages/rest_framework/fields.py", line 788, in to_representation
    return six.text_type(value)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2699: ordinal not in range(128)

Did not have the time to investigate any further right now (i.e. if it is related to the translation), but for me it seems to be a classical python2 utf-8-decode error.

BTW: I also have made a join request as a German and French translator (being bi-lingual)...