Cannot import name 'Item' from partially initialized module 'vdirsyncer.vobject'

• Your vdirsyncer version
  vdirsyncer, version 0.16.9.dev0+gb5dd092.d20201204

• Your Python version
  Python 3.9.1

• Your config file

[general]
status_path = "~/.local/share/vdirsyncer/status/"

[pair nc_contacts]
a = "nc_contacts_local"
b = "nc_contacts_remote"
collections = ["from a", "from b"]
conflict_resolution = "b wins"

[storage nc_contacts_local]
type = "filesystem"
path = ...
fileext = ".vcf"

[storage nc_contacts_remote]
type = "carddav"

url = ...
username =  ...
password.fetch = ...

[pair nc_calendar]
a = "nc_calendar_local"
b = "nc_calendar_remote"
collections = ["from a", "from b"]
metadata = ["color"]
conflict_resolution = "b wins"

[storage nc_calendar_local]
type = "singlefile"
path = ...

[storage nc_calendar_remote]
type = "caldav"
url = ...
username =  ...
password.fetch = ...

• Your operating system
  Manjaro-ARM

My configuration consists of a Nextcloud address book and a Nextcloud calendar. When executing:

 vdirsyncer sync

I usually get an error. But I have also tried this:

vdirsyncer sync --max-workers=1

and I have not faced any error yet.

E.g:

[kde@plasma-mobile ~]$ vdirsyncer sync 
Syncing nc_contacts/contacts
Syncing nc_calendar/personal
error: Unknown error occurred for nc_calendar/personal: cannot import name 'Item' from partially initialized module 'vdirsyncer.vobject' (most likely due to a circular import) (/usr/lib/python3.9/site-packages/vdirsyncer/vobject.py)
error: Use `-vdebug` to see the full traceback.
error: 1 out of 4 tasks failed.
[kde@plasma-mobile ~]$ vdirsyncer sync --max-workers=1
Syncing nc_contacts/contacts
Syncing nc_calendar/personal
[kde@plasma-mobile ~]$ 

Reproduce the failure with the reported vdirsyncer configuration using vdirsyncer sync, then compare it with --max-workers=1. Start with the vdirsyncer.vobject module shown in the error and use -vdebug to obtain the full traceback. Done means concurrent synchronization no longer produces the partially initialized Item import error.