On occasion I have exported the pool "tank" and later reimported. This always worked in 11.2 releases but fails now in 11.e-U1. I get an error message indicating thatthe mount poibt is not available. Here's a log:

Error: 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 43, in _call
return methodobj(*params)
File "/usr/local/lib/python3.7/site-packages/middlewared/worker.py", line 43, in _call
return methodobj(*params)
File "/usr/local/lib/python3.7/site-packages/middlewared/schema.py", line 965, in nf
return f(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/middlewared/plugins/zfs.py", line 382, in import_pool
zfs.import_pool(found, found.name, options, any_host=any_host)
File "libzfs.pyx", line 369, in libzfs.ZFS._exit_
File "/usr/local/lib/python3.7/site-packages/middlewared/plugins/zfs.py", line 382, in import_pool
zfs.import_pool(found, found.name, options, any_host=any_host)
File "libzfs.pyx", line 878, in libzfs.ZFS.import_pool
libzfs.ZFSException: failed to create mountpoint
"""

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/job.py", line 349, in run
await self.future
File "/usr/local/lib/python3.7/site-packages/middlewared/job.py", line 386, in __run_body
rv = await self.method(*([self] + args))
File "/usr/local/lib/python3.7/site-packages/middlewared/schema.py", line 961, in nf
return await f(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/middlewared/plugins/pool.py", line 1927, in import_pool
'cachefile': ZPOOL_CACHE_FILE,
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))
libzfs.ZFSException: ('failed to create mountpoint',)

Only way I could get my pool imported again was to upload a config (in which the pool was already imported).