error: Unknown error occurred for calendars

Hi,

I receive following error since today. Let me know if there is any more information I can provide.

• Your vdirsyncer version

[~]$ vdirsyncer --version
vdirsyncer, version 0.16.9.dev0+gb5dd092.d20200612

• If applicable, which server software (and which version) you're using
  Fastmail
• Your Python version

[~]$ python --version
Python 3.8.3

• Your operating system

Arch Linux

• Your config file

[~]$ cat .config/vdirsyncer/config
[general]
status_path = "~/.vdirsyncer/status"

[pair contacts]
a = "contacts_local"
b = "contacts_remote"
collections = ["from a", "from b"]

[pair calendars]
a = "calendars_local"
b = "calendars_remote"
collections = ["from a", "from b"]

[storage contacts_local]
type = "filesystem"
path = "~/.contacts"
fileext = ".vcf"

[storage contacts_remote]
type = "carddav"
url = "https://carddav.fastmail.com/"
username = "..."
password.fetch  = ["command", "/home/radek/bin/getpw.py", "...", "..."]

[storage calendars_local]
type = "filesystem"
path = "~/.calendars"
fileext = ".ics"

[storage calendars_remote]
type = "caldav"
url = "https://caldav.fastmail.com/"
username = "..."
password.fetch  = ["command", "/home/radek/bin/getpw.py", "...", "..."]

• Use vdirsyncer -vdebug for debug output. The output is sensitive, but
  please attach at least the last few lines before the error (if applicable),
  censored as necessary. This is almost always the most useful
  information.

error: Unknown error occurred for calendars/...: /dav/calendars/user/.../.../...
error: Use `-vdebug` to see the full traceback.
debug:   File "/usr/lib/python3.8/site-packages/vdirsyncer/cli/tasks.py", line 62, in sync_collection
debug:     sync.sync(
debug:   File "/usr/lib/python3.8/site-packages/vdirsyncer/sync/__init__.py", line 135, in sync
debug:     b_nonempty = b_info.prepare_new_status()
debug:   File "/usr/lib/python3.8/site-packages/vdirsyncer/sync/__init__.py", line 57, in prepare_new_status
debug:     for href, item, etag in (self.storage.get_multi(prefetch)
debug:   File "/usr/lib/python3.8/site-packages/vdirsyncer/storage/dav.py", line 503, in get_multi
debug:     raise exceptions.NotFoundError(href)
error: 1 out of 13 tasks failed.

Start by reproducing the calendar sync with vdirsyncer -vdebug and inspect the traceback through vdirsyncer/cli/tasks.py, vdirsyncer/sync/__init__.py, and vdirsyncer/storage/dav.py. Trace the NotFoundError raised by get_multi and determine why the Fastmail calendar item cannot be retrieved. Done means the reported calendar sync completes without the unknown error and the cause is covered by an appropriate regression check.