If you change from SSL to STARTTLS or similar, you need to akonadictl restart, which is not very use friendly. Simply pressing Ok/Apply on the user interface should work.
A possibly relevant merge request was started @ https://invent.kde.org/pim/kdepim-runtime/-/merge_requests/64
Git commit f88cf86d696596a157c99c900243b621ae45f3c2 by Laurent Montel, on behalf of Albert Astals Cid. Committed on 16/12/2021 at 05:41. Pushed by mlaurent into branch 'release/21.12'. pop3: Properly reload settings on configuration change M +1 -0 resources/pop3/pop3resource.cpp https://invent.kde.org/pim/kdepim-runtime/commit/f88cf86d696596a157c99c900243b621ae45f3c2