Searching results in "Could not open inbox" most of the time (approx. 9/10 cases)

Steps to reproduce

1. Go to inbox
2. Type something into the search input field
3. Wait (approx. 30s)
4. Get error message "Could not open inbox" most of the time (approx. 9/10 cases)

Expected behavior

See the actual mails which match the search.

Actual behavior

I get the message "Could not open inbox".

Mail app version

3.6.1

Mailserver or service

Dovecot 2.3.20

Operating system

GNU/Linux

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database

PostgreSQL

Additional info

I assume this is some kind of timeout related problem. In the least of cases (i.e. one out of ten attempts), I get the expected search result quite fast (after 2-3 seconds). However, most of the time I only get "could not open inbox" after approx. 30s of waiting.

Nextcloud Mail and Dovecot are hosted on the same server and communicating via the loopback interface. So, network latency should not be part of the problem. Also, the server is rather potent (AMD EPYC 7702P, 128G RAM).

When I run the same query via IMAP from another mail client (Thunderbird on Windows, KMail on Linux), I always receive the search result after a couple of seconds. And there is actual network communication involved between my home ISP and the hoster.

Reproduce a search from the inbox using the reported steps, including the approximately 30-second wait, and compare the failing result with the successful 2–3 second attempts. Trace the inbox search flow and its communication with Dovecot; done means matching mails are returned reliably instead of showing "Could not open inbox".