Can't login to DigitalOcean's droplets after restart

Ubuntu Server 18.04, droplet on DigitalOcean, works many years as a web server. UFW with some usual rules is installed. Problem: after restarting the server, I can't log in with SSH. There is simply no login prompt, the screen is blank. But - if I than open the Recovery Console (on DigitalOcean), I can log in (as expected). Then I use "ufw disable" and "ufw enable" and log out from Recovery Console. After that, I can always log in with SSH and work as normal. I can't find anything suspicious in log file. Any idea what to check?