icloud – Contact cards vanished. After Big Sur update, Contacts won’t open at all

OSX Contacts app started to act strangely a while ago. It opened and showed all the contact sources/accounts but none of these would display actual contact cards. Contact autofill functionality remained in Mail. Contacts are still visible on iPhone and through icloud.com. I tried various fixes without luck, including trashing Library/ApplicationSupport/AddressBook. Nothing worked so I decided to finally do the overdue update to Big Sur, hoping that the fresh install of Contacts would fix the problem. After the update, the contents of Library/ApplicationSupport/AddressBook look healthy but Contacts won’t open at all. When I try to send the error report to Apple, it freezes and I have to close the process through Activity Monitor. The Big Sur update also brought the added bonus of none of my System Preference panes opening anymore but I don’t know if this is related to the Contacts problem. Any ideas, please? I’m clueless at this point. Thanks!