understanding python stacktrace in mailman3 log

Question

I just upgraded a postorius/mailman3 instance. Now the webfrontend shows HTTP Error 500: {"title": "500 Internal Server Error"} and the mailman3 log contains this message:

  File "falcon/app.py", line 365, in falcon.app.App.__call__
  File "/usr/lib/python3/dist-packages/mailman/rest/lists.py", line 341, in on_get
    resource = self._make_collection(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mailman/rest/helpers.py", line 171, in _make_collection
    start, total_size, collection = self._paginate(
                                    ^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mailman/rest/helpers.py", line 167, in _paginate
    return list_start, total_size, collection[list_start:list_end]
                                   ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mailman/utilities/queries.py", line 63, in __getitem__
    .all()
     ^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 1476, in all
    return self._allrows()
           ^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 401, in _allrows
    rows = self._fetchall_impl()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 1389, in _fetchall_impl
    return self._real_result._fetchall_impl()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 1813, in _fetchall_impl
    return list(self.iterator)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/orm/loading.py", line 147, in chunks
    fetch = cursor._raw_all_rows()
            ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 393, in _raw_all_rows
    return [make_row(row) for row in rows]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/engine/result.py", line 393, in <listcomp>
    return [make_row(row) for row in rows]
            ^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sqlalchemy/sql/sqltypes.py", line 1870, in process
    return loads(value)
           ^^^^^^^^^^^^
_pickle.UnpicklingError: invalid load key, '\xc2'.

What does this mean? The versions in use are the most current packages from the Debian bookworm repository. where would I start to search here? What is happening in this Stacktrace?

I would usually ask such things on Server Fault, but it seems this error needs some knowledge about the frameworks and libraries in use.