[RHUI 3] Syncing RHEL 8 Appstream repository failed with 'Error: strings in documents must be valid UTF-8' or 'Importer indicated a failed response'

Issue

• Syncing RHEL 8 Appstream repository failed, its task detail is as follows:

:
(snip)
Metadata:
      State: FINISHED
    Modules:
      Error: strings in documents must be valid UTF-8:
             '\x82\x00\x00\x00\x041.16\x00\x13\x00\x00\x00\x020\x00\x07\x00\x00\
             x00common\x00\x00\x041.22\x00\x13\x00\x00\x00\x020\x00\x07\x00\x00\
             x00common\x00\x00\x041.14\x00\x13\x00\x00\x00\x020\x00\x07\x00\x00\
             x00common\x00\x00\x041.20\x00\x13\x00\x00\x00\x020\x00\x07\x00\x00\
             x00common\x00\x00\x041.18\x00\x13\x00\x00\x00\x020\x00\x07\x00\x00\
             x00common\x00\x00\x00'
      State: FAILED
    Purge Duplicates:
      State: NOT_STARTED
Traceback:        Traceback (most recent call last):   File
                  "/usr/lib/python2.7/site-packages/celery/app/trace.py", line
                  367, in trace_task     R = retval = fun(*args, **kwargs)
                  File
                  "/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py",
                  line 529, in __call__     return super(Task,
                  self).__call__(*args, **kwargs)   File
                  "/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py",
                  line 107, in __call__     return super(PulpTask,
                  self).__call__(*args, **kwargs)   File
                  "/usr/lib/python2.7/site-packages/celery/app/trace.py", line
                  622, in __protected_call__     return self.run(*args,
                  **kwargs)   File
                  "/usr/lib/python2.7/site-packages/pulp/server/controllers/repo
                  sitory.py", line 828, in sync     raise
                  pulp_exceptions.PulpExecutionException(_('Importer indicated a
                  failed response')) PulpExecutionException: Importer indicated
                  a failed response