acme-v02.api is still serving a chain rooted in DST Root CA X3 for now, though of course I'm curious when it'll change to just be ISRG Root X1.
Had this server been able to connect before, but can't now? I'm assuming the problem isn't the API endpoint using the "long" chain now, though I suspect that's the only thing that's changed on the Let's Encrypt side of things in the past couple months.