mail alerts pool as OFFLINE except it's not

I get frequent mail alerts, claiming my pool ist OFFLINE. When I check the GUI nothing seems to be offline. Below is an example email text, the messages differ from time to time. I also get alarms for my main pool claiming it would be offline.

I am unable to figure out what is causing these messages.

Example email text:

FreeNAS @ freenas.WORKGRP

The following alert has been cleared:

• Failed to check for alert BootPoolStatus:
  concurrent.futures.process._RemoteTraceback:
  """
  Traceback (most recent call last):
  File "/usr/local/lib/python3.7/concurrent/futures/process.py", line 239, in _process_worker
  r = call_item.fn(*call_item.args, **call_item.kwargs)
  File "/usr/local/lib/python3.7/site-packages/middlewared/worker.py", line 95, in main_worker
  res = loop.run_until_complete(coro)
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 579, in run_until_complete
  return future.result()
  File "/usr/local/lib/python3.7/site-packages/middlewared/worker.py", line 51, in _run
  return await self._call(name, serviceobj, methodobj, params=args, job=job)
  File "/usr/local/lib/python3.7/site-packages/middlewared/worker.py", line 34, in _call
  with Client('ws+unix:///var/run/middlewared-internal.sock', py_exceptions=True) as c:
  File "/usr/local/lib/python3.7/site-packages/middlewared/client/client.py", line 360, in _init_
  self._ws.connect()
  File "/usr/local/lib/python3.7/site-packages/middlewared/client/client.py", line 181, in connect
  rv = super(WSClient, self).connect()
  File "/usr/local/lib/python3.7/site-packages/ws4py/client/_init_.py", line 239, in connect
  self.protocols, self.extensions = self.process_handshake_header(headers)
  File "/usr/local/lib/python3.7/site-packages/ws4py/client/_init_.py", line 332, in process_handshake_header
  raise HandshakeError("Invalid challenge response: %s" % value)
  ws4py.exc.HandshakeError: Invalid challenge response: b'3chy7ewd0p3reftzzk0xpfrygm4='
  """

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/middlewared/plugins/alert.py", line 650, in __run_source
alerts = (await alert_source.check()) or []
File "/usr/local/lib/python3.7/site-packages/middlewared/plugins/../alert/source/boot_pool.py", line 15, in check
pool = await self.middleware.call("zfs.pool.query", [["id", "=", "freenas-boot"]])
File "/usr/local/lib/python3.7/site-packages/middlewared/main.py", line 1127, in call
app=app, pipes=pipes, job_on_progress_cb=job_on_progress_cb, io_thread=True,
File "/usr/local/lib/python3.7/site-packages/middlewared/main.py", line 1074, in _call
return await self._call_worker(name, *args)
File "/usr/local/lib/python3.7/site-packages/middlewared/main.py", line 1094, in _call_worker
return await self.run_in_proc(main_worker, name, args, job)
File "/usr/local/lib/python3.7/site-packages/middlewared/main.py", line 1029, in run_in_proc
return await self.run_in_executor(self.__procpool, method, *args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/middlewared/main.py", line 1003, in run_in_executor
return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
ws4py.exc.HandshakeError: Invalid challenge response: b'3chy7ewd0p3reftzzk0xpfrygm4='

Current alerts:

• Scrub of pool 'freenas-boot' finished.