matrixsynapse: TypeError: load() missing 1 required positional argument: 'Loader'

In testing container, with all package updates installed, I tried to install Matrix Synapse:

    INFO plinth.actions       # matrixsynapse..post_install(…) 
   ERROR plinth.actions       Error running action matrixsynapse..post_install(*(), **{}): ("load() missing 1 required positional argument: 'Loader'", b'', b'\x1b[31m   ERROR\x1b[0m \x1b[94m__main__            \x1b[0m Error executing action: load() missing 1 required positional argument: \'Loader\'\nTraceback (most recent call last):\n  File "/freedombox/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/freedombox/plinth/modules/matrixsynapse/privileged.py", line 52, in post_install\n    orig_config = yaml.load(orig_conf_file)\n                  ^^^^^^^^^^^^^^^^^^^^^^^^^\nTypeError: load() missing 1 required positional argument: \'Loader\'\n') ("load() missing 1 required positional argument: 'Loader'", b'', b'\x1b[31m   ERROR\x1b[0m \x1b[94m__main__            \x1b[0m Error executing action: load() missing 1 required positional argument: \'Loader\'\nTraceback (most recent call last):\n  File "/freedombox/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/freedombox/plinth/modules/matrixsynapse/privileged.py", line 52, in post_install\n    orig_config = yaml.load(orig_conf_file)\n                  ^^^^^^^^^^^^^^^^^^^^^^^^^\nTypeError: load() missing 1 required positional argument: \'Loader\'\n') ['  File "/freedombox/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n', '  File "/freedombox/plinth/modules/matrixsynapse/privileged.py", line 52, in post_install\n    orig_config = yaml.load(orig_conf_file)\n                  ^^^^^^^^^^^^^^^^^^^^^^^^^\n']
    INFO plinth.setup         Setup completed: matrixsynapse: Error installing app: ("load() missing 1 required positional argument: 'Loader'", b'', b'\x1b[31m   ERROR\x1b[0m \x1b[94m__main__            \x1b[0m Error executing action: load() missing 1 required positional argument: \'Loader\'\nTraceback (most recent call last):\n  File "/freedombox/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/freedombox/plinth/modules/matrixsynapse/privileged.py", line 52, in post_install\n    orig_config = yaml.load(orig_conf_file)\n                  ^^^^^^^^^^^^^^^^^^^^^^^^^\nTypeError: load() missing 1 required positional argument: \'Loader\'\n') ("load() missing 1 required positional argument: 'Loader'", b'', b'\x1b[31m   ERROR\x1b[0m \x1b[94m__main__            \x1b[0m Error executing action: load() missing 1 required positional argument: \'Loader\'\nTraceback (most recent call last):\n  File "/freedombox/actions/actions", line 92, in _call\n    return_values = func(*arguments[\'args\'], **arguments[\'kwargs\'])\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/freedombox/plinth/modules/matrixsynapse/privileged.py", line 52, in post_install\n    orig_config = yaml.load(orig_conf_file)\n                  ^^^^^^^^^^^^^^^^^^^^^^^^^\nTypeError: load() missing 1 required positional argument: \'Loader\'\n')
   DEBUG plinth.operation     Operation: matrixsynapse: Installing app: on_complete called