Thanks a lot for completing your answer! ☺
I’ve tried to disable HTTP/2 on my server like this:
server { # listen 443 ssl http2; listen 443 ssl; server_name dav.mydomain.tld; root /srv/http/sabredav/; [...]…but after further investigations today, I oddly found that my server was still considered as HTTP/2 enabled for dav.mydomain.tld… (test of domain with https://tools.keycdn.com/http2-test )
I will try soon to disable http/2 everywhere in nginx, to see if it works, and follow the issue with okhttp.