@schwa@mastodon.social Do you still have this problem?
I ran into it after restoring a full backup from Time Machine to a new Mac; Sequoia on both source and target. Reminders was entirely blank on my new Mac (no entries in the sidebar, like in your case).
This helped me (the post that I link to, and the one that it quotes): https://forums.macrumors.com/threads/reminders-app-on-mac-blank-after-migrating-to-new-mac.2393387/post-33856362
The main difference was that I killed “remindd” in Activity Monitor *before* deleting the directories, to be sure that it didn’t try a sync before I rebooted.