It seems like it fixed my problems with help from the Onlyoffice forum! But I will update here too so that others can maybe get some idea’s from this.

Onlyoffice:
default.json -> “rejectUnauthorized” to false.
local.json -> “header”: “AuthorizationJwt” from “header”: “Authorization”
supervisorctl restart all

Nexcloud:
config/config.php ->
‘onlyoffice’ => array (
‘verify_peer_off’ => true,
‘jwt_header’ => “AuthorizationJwt”
)