I guess apple doesn't want me to use Reminders on the Mac any more.
Reminders shows only blank window in Mac… - Apple Community

@schwa It feels like anyone who knew how to do UI retired, and the new team said, let's rewrite the whole thing and shoot for maximum backwards compatibility "as speced" so we can clear that 25 year UI backlog at the same time.

I'm not a dev manager, but I've been managed, and this is what it feels like

@schwa Hrm, feels like I see more posts like this in the subreddits lately too. Everything gets wiped out from Reminders and/or sometimes Notes too. What's going on over there.
@chartier They fuck with the rounded rects and the rounded rects are fucking back.
@chartier @schwa There's been reports of CloudKit not working with 26.4: https://developer.apple.com/forums/thread/820550 That thread talks about the breakage happening on iOS, not Mac, but might be related anyway.
iCloud Sync not working with iPhon… | Apple Developer Forums

@schwa 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.

Reminders App on Mac Blank after Migrating to new Mac

I had this same problem and can confirm that kirbyrun's solution worked for me, i.e. emptying the /Users/USER NAME/Library/Reminders folder was not the answer. In the Console app there were error messages generated when opening the Reminders app. For example: error: (11) Fatal error. The...

MacRumors Forums
@johninparis I fixed it by various nuking of the settings and rebooting.

@schwa That’ll do it! 😜 I don’t know my way around all of the various container directories, so my nuking wasn’t successful until I’d done it in the places that those people listed in their posts.

I pine for the days when holding down the Option key revealed in-app reset functions. It was still a non-obvious thing to do, but it at least avoided having to wade through a potential minefield of preference and state files.

I’m glad that you were able to work it out!