… cannot be cast to net.fortuna.ical4j.model.Property.Uid
Looks like a bug to me.
Duplicate of https://forums.bitfire.at/post/11619?
Thanks for your report.
Duplicate of #170, please follow up there.
#578 is caused by https://github.com/owncloud/contacts/issues/581 which is present since at least OwnCloud 6.0.3.
Please follow up in the issues mentioned above.
Same as https://forums.bitfire.at/post/8531, will be fixed in next version