I think the problem stems from the fact that Mailbox.org use popup dialogues extensively in their webmail.
I didn't test the entire interface but I couldn't load the popup filter dialogue and in settings the options for filtering and inbox encryption were missing until I added mailbox.org to the HTTPS filtering whitelist.
It may be worthwhile to add this to the whitelist by default, along with the others there.
Android 10
Bromite/Ungoogled Chromium
Beta, (Latest)
I didn't test the entire interface but I couldn't load the popup filter dialogue and in settings the options for filtering and inbox encryption were missing until I added mailbox.org to the HTTPS filtering whitelist.
It may be worthwhile to add this to the whitelist by default, along with the others there.
Android 10
Bromite/Ungoogled Chromium
Beta, (Latest)