Also encountered this problem while trying to set values on a router.  Running the command:

    openstack router set --external-gateway provider_network external

resulted in the "BadRequestException: Unknown error" message.


Adding a "--debug" switch (openstack --debug router set --external-gateway provider_network external) managed to show the error, which led to a solution.  But it would have been better to get more descriptive output at the beginning.


RELEVANT OUTPUT NEAR THE ERROR:
===========================================================================

GET call to network for http://192.168.77.69:9696/v2.0/networks used request id req-cae8f31f-00a3-4ef8-b3b8-cb6c6125344a
Manager unknown ran task network.GET.networks in 0.112754106522s
Manager unknown running task network.PUT.routers
REQ: curl -g -i -X PUT http://192.168.77.69:9696/v2.0/routers/56479717-b56c-44c9-af6a-1f031219941b -H "User-Agent: osc-lib/1.9.0 keystoneauth1/3.4.0 python-requests/2.14.2 CPython/2.7.5" -H "Content-Type: application/json" -H "X-Auth-Token: {SHA1}5a9c2739672fb9878895b14d461c4b1438ec5713" -d '{"router": {"external_gateway_info": {"network_id": "78a1c6f7-fe5a-4ba0-99b0-c66b03c8ceec"}}}'
http://192.168.77.69:9696 "PUT /v2.0/routers/56479717-b56c-44c9-af6a-1f031219941b HTTP/1.1" 400 161
RESP: [400] Content-Type: application/json Content-Length: 161 X-Openstack-Request-Id: req-c6ce077c-771a-4f8d-b205-df4720530d60 Date: Thu, 27 Sep 2018 20:19:06 GMT 
RESP BODY: {"NeutronError": {"message": "Bad router request: Network 78a1c6f7-fe5a-4ba0-99b0-c66b03c8ceec is not an external network.", "type": "BadRequest", "detail": ""}}

PUT call to network for http://192.168.77.69:9696/v2.0/routers/56479717-b56c-44c9-af6a-1f031219941b used request id req-c6ce077c-771a-4f8d-b205-df4720530d60
Manager unknown ran task network.PUT.routers in 0.741237878799s
BadRequestException: Unknown error



Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 400, in run_subcommand
    result = cmd.run(parsed_args)
  File "/usr/lib/python2.7/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/cliff/command.py", line 184, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python2.7/site-packages/openstackclient/network/v2/router.py", line 642, in take_action
    client.update_router(obj, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/network/v2/_proxy.py", line 2316, in update_router
    return self._update(_router.Router, router, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 38, in check
    return method(self, expected, actual, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 174, in _update
    return res.update(self)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 820, in update
    self._translate_response(response, has_body=has_body)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 664, in _translate_response
    exceptions.raise_from_response(response, error_message=error_message)
  File "/usr/lib/python2.7/site-packages/openstack/exceptions.py", line 205, in raise_from_response
    http_status=http_status, request_id=request_id
BadRequestException: BadRequestException: Unknown error
clean_up SetRouter: BadRequestException: Unknown error
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 134, in run
    ret_val = super(OpenStackShell, self).run(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 279, in run
    result = self.run_subcommand(remainder)
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 169, in run_subcommand
    ret_value = super(OpenStackShell, self).run_subcommand(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 400, in run_subcommand
    result = cmd.run(parsed_args)
  File "/usr/lib/python2.7/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/cliff/command.py", line 184, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python2.7/site-packages/openstackclient/network/v2/router.py", line 642, in take_action
    client.update_router(obj, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/network/v2/_proxy.py", line 2316, in update_router
    return self._update(_router.Router, router, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 38, in check
    return method(self, expected, actual, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 174, in _update
    return res.update(self)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 820, in update
    self._translate_response(response, has_body=has_body)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 664, in _translate_response
    exceptions.raise_from_response(response, error_message=error_message)
  File "/usr/lib/python2.7/site-packages/openstack/exceptions.py", line 205, in raise_from_response
    http_status=http_status, request_id=request_id
BadRequestException: BadRequestException: Unknown error