Mail.app search stopped working

Hi everyone,

since the upgrade from Catalina to Big Sur, Mail.app search within email messages stopped working for me.

Here's what's happening:

• If I type a search term into the upper right input field, the word "Searching..." appears at the bottom of the column where the mail topics are displayed, but it never returns any search results
• There's no auto-completion of the search term anymore - I seem to remember that in Catalina, I got suggestions for mail addresses when I started typing and a dropdown with options like "To", "From" and "Whole Text" or something like that. That's not happening anymore.
• I don't see mail messages as results in Spotlight search anymore

Here's what I tried / checked:

• Made sure the Mail folder is included in Spotlight
• Rebuilt the Spotlight index as described in https://support.apple.com/en-us/HT201716, once for my Mail folder only, then for my complete account
• Used the "Mailbox -> Rebuild" option in Mail.app for all mailboxes
• Deleted the configured mail accounts and set them up from scratch (deleting ~/Library/Mail in between)
• Arbitrary combinations of the above points

If anybody has an idea what else to try, please let me know.

Thanks,

Marc