Maas with RBAC fails random commands with: Expecting value: line 1 column 1 (char 0)

I have a Maas deployment setup with RBAC configauth. I'm having some problems with the stability of this configuration, arbitrary commands from both Maas and the juju maas controller end with `500 Internal Server Error (Expecting value: line 1 column 1 (char 0))`, e.g.:
```
2022-06-16-12:41:08 root ERROR [localhost] Command failed: juju bootstrap --bootstrap-constraints 'arch=amd64 tags=juju ' --config bootstrap-timeout=1800 --model-default /home/ubuntu/project/generated/model_defaults.yaml maas_cloud foundations-maas
2022-06-16-12:41:08 root ERROR [localhost] STDOUT follows:

2022-06-16-12:41:08 root ERROR [localhost] STDERR follows:
Creating Juju controller "foundations-maas" on maas_cloud
Looking for packaged Juju agent version 2.9.31 for amd64
Located Juju agent version 2.9.31-ubuntu-amd64 at https://streams.canonical.com/juju/tools/agent/2.9.31/juju-2.9.31-linux-amd64.tgz
Launching controller instance(s) on maas_cloud...
ERROR failed to bootstrap model: cannot start bootstrap instance: unexpected: ServerError: 500 Internal Server Error (Expecting value: line 1 column 1 (char 0))
```

In this particular command I am trying to bootstrap the juju maas controller. It fails most of the time on different places in the bootstrapping process with the same error. I did manage to successfully bootstrap once, but then juju failed again on this error when deploying a bundle.

Another example (on the maas host):
```
ubuntu@infra1$ maas root vm-hosts read type=virsh --debug

500 Internal Server Error

   Content-Length: 41
     Content-Type: text/plain; charset=utf-8
             Date: Thu, 16 Jun 2022 13:17:23 GMT
           Server: TwistedWeb/18.9.0
           Status: 500
             Vary: Cookie
  X-Frame-Options: SAMEORIGIN

Expecting value: line 1 column 1 (char 0)
```
And the same on a remote machine:
```
ubuntu@lab3-rufflet$ maas root vm-hosts read type=virsh --debug
500 Internal Server Error

   Content-Length: 41
     Content-Type: text/plain; charset=utf-8
             Date: Thu, 16 Jun 2022 13:14:40 GMT
           Server: TwistedWeb/18.9.0
           Status: 500
             Vary: Cookie
  X-Frame-Options: SAMEORIGIN

Expecting value: line 1 column 1 (char 0)
```
These errors happen only 1 in every ~10 commands.

The error message sounds like Maas is getting an empty response on a requests, I assume the response comes from the RBAC server because we do not have these problems when local authentication is used.

In the maas log:
```
regiond.log-2022-06-16 13:15:40 regiond: [info] 10.244.40.1 GET /MAAS/api/2.0/vm-hosts/?type=virsh HTTP/1.1 --> 500 INTERNAL_SERVER_ERROR (referrer: -; agent: Python-httplib2/0.14.0 (gzip))
regiond.log-2022-06-16 13:15:43 regiond: [info] 10.244.40.1 GET /MAAS/api/2.0/vm-hosts/?type=virsh HTTP/1.1 --> 200 OK (referrer: -; agent: Python-httplib2/0.14.0 (gzip))
regiond.log-2022-06-16 13:16:07 regiond: [info] 10.244.40.30 GET /MAAS/rpc/ HTTP/1.1 --> 200 OK (referrer: -; agent: provisioningserver.rpc.clusterservice.ClusterClientService)
regiond.log-2022-06-16 13:17:07 regiond: [info] 10.244.40.30 GET /MAAS/rpc/ HTTP/1.1 --> 200 OK (referrer: -; agent: provisioningserver.rpc.clusterservice.ClusterClientService)
regiond.log-2022-06-16 13:17:14 regiond: [info] 10.244.40.1 GET /MAAS/api/2.0/vm-hosts/?type=virsh HTTP/1.1 --> 200 OK (referrer: -; agent: Python-httplib2/0.14.0 (gzip))
regiond.log-2022-06-16 13:17:18 regiond: [info] 10.244.40.1 GET /MAAS/api/2.0/vm-hosts/?type=virsh HTTP/1.1 --> 200 OK (referrer: -; agent: Python-httplib2/0.14.0 (gzip))
regiond.log-2022-06-16 13:17:20 regiond: [info] 10.244.40.1 GET /MAAS/api/2.0/vm-hosts/?type=virsh HTTP/1.1 --> 200 OK (referrer: -; agent: Python-httplib2/0.14.0 (gzip))
regiond.log:2022-06-16 13:18:23 maasserver: [error] ################################ Exception: Expecting value: line 1 column 1 (char 0) ################################
regiond.log:2022-06-16 13:18:23 maasserver: [error] Traceback (most recent call last):
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 113, in _get_response
regiond.log- response = wrapped_callback(request, *callback_args, **callback_kwargs)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/utils/views.py", line 284, in view_atomic_with_post_commit_savepoint
regiond.log- return view_atomic(*args, **kwargs)
regiond.log- File "/usr/lib/python3.8/contextlib.py", line 75, in inner
regiond.log- return func(*args, **kwds)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/api/support.py", line 56, in __call__
regiond.log- response = super().__call__(request, *args, **kwargs)
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/django/views/decorators/vary.py", line 20, in inner_func
regiond.log- response = func(*args, **kwargs)
regiond.log- File "/snap/maas/19835/usr/lib/python3.8/dist-packages/piston3/resource.py", line 197, in __call__
regiond.log: result = self.error_handler(e, request, meth, em_format)
regiond.log- File "/snap/maas/19835/usr/lib/python3.8/dist-packages/piston3/resource.py", line 195, in __call__
regiond.log- result = meth(request, *args, **kwargs)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/api/support.py", line 308, in dispatch
regiond.log- return function(self, request, *args, **kwargs)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/api/pods.py", line 464, in read
regiond.log- return Pod.objects.get_pods(request.user, PodPermission.view).order_by(
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/models/bmc.py", line 601, in get_pods
regiond.log- fetched = rbac.get_resource_pool_ids(
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/rbac.py", line 321, in get_resource_pool_ids
regiond.log- results = self._get_resource_pool_identifiers(user, *permissions)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/rbac.py", line 375, in _get_resource_pool_identifiers
regiond.log- fetched = self.client.allowed_for_user(
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/rbac.py", line 139, in allowed_for_user
regiond.log- result = self._request("GET", url)
regiond.log- File "/snap/maas/19835/lib/python3.8/site-packages/maasserver/macaroon_auth.py", line 291, in _request
regiond.log- content = resp.json()
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/requests/models.py", line 897, in json
regiond.log- return complexjson.loads(self.text, **kwargs)
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/simplejson/__init__.py", line 518, in loads
regiond.log- return _default_decoder.decode(s)
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/simplejson/decoder.py", line 370, in decode
regiond.log- obj, end = self.raw_decode(s)
regiond.log- File "/snap/maas/19835/usr/lib/python3/dist-packages/simplejson/decoder.py", line 400, in raw_decode
regiond.log- return self.scan_once(s, idx=_w(s, idx).end())
regiond.log:simplejson.errors.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

On the rbac server, I am seeing some database errors:
```
postgresql/postgresql-10-main.log:2022-06-16 12:51:07.225 UTC [14754] rbac@rbac ERROR: could not serialize access due to concurrent update
postgresql/postgresql-10-main.log-2022-06-16 12:51:07.225 UTC [14754] rbac@rbac STATEMENT: UPDATE service SET last_contact='2022-06-16T12:51:07.190373'::timestamp WHERE service.id = 1
```

But the occurrences of these messages data base errors does not seem to correlate to the ServerErrors.

The maas snap logs are in the attachments.