thunderbird-android: K9mail can't find public keys of recipients

Expected behavior

Successfully encrypt messages for recipients with an associated key in Openkeychain (OKC).

Actual behavior

When I try to send encrypted messages to recipients with a public key in OKC I get this :

"private mode is enabled but some recipients don't have keys." and the dots of the lock icon don’t become green anymore.

I’ve triple-checked the crypto settings of the account and the recipient address of course.

It used to work before but not anymore, I don’t know what triggered that behavior.

Steps to reproduce

1. create a new mail, fill recipient
2. Select Encrypt
3. Tap on the send icon

Environment

K-9 Mail version: 5.203 from fdroid

Android version: cm-13

Account type : POP3