I am running 3.5.0.beta8-dev with Full (strict) SSl with Cloudflare and do not have any issues.
A couple of things to check:
- Check your Let’s Encrypt - Review your upgrade log and validate everything was rebuilt correctly.
- Check for any external firewall changes (DigitalOcean provides external firewalls)
The 521 and switching to Flexible indicates your server is not set up with HTTPS.