When one of the syncing addresses is unreachable other servers are not synchronized

[Janovlk]

Hello,

Generally the Vdirsyncer work reliably. What derails it completely is when one of the syncing servers is unreachable - not necessarily offline. I defined three syncing pairs. One "production" CalDAV/CardDAV server is backed-up to three other servers. When one of those backup servers is offline, the sync fails with two others too, but the error message
Unknown error occurred for carddav_tabdigital: 503, message='Service Unavailable', url='https://DAV.server.tld/remote.php/dav/addressbooks/users/user/contacts/'
is reported once and for only one of offline services (CardDAV).
One of backup servers is Radicale with comprehensive logging where I can see the Vdirsync logs in but no synchronization occurs.
This phenomenon occurs in versions 0.19.3 and 0.20.

PP