Proxmox web interface is not accessible

Question

I have a PC with Proxmox. I have been using it half year ago. It was 5.4 version. I've started it yesterday, but couldn't connect to it using webinterface: 192.168.1.21:8006. Chrome said: ERR_EMPTY_RESPONSE
Looking for solution, I've found it could be solved by

1) Upgrade. I upgraded from 5.4 to 6.1 and it did not resolved the issue
2) Reset certificates: pvecm updatecerts -f. It did not resolved the issue. 3) Clear browser's cookies. There were no cookies. I've also used Chrome's incognito mode and different browsers that never been connected to my Proxmox server.

root@proxmox:~# netstat -na | grep 8006
tcp        0      0 0.0.0.0:8006            0.0.0.0:*               LISTEN

root@proxmox:~# pveversion
pve-manager/6.1-7/13e58d5e (running kernel: 5.3.13-3-pve)

root@proxmox:~# systemctl status pveproxy
● pveproxy.service - PVE API Proxy Server
   Loaded: loaded (/lib/systemd/system/pveproxy.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2020-02-07 23:18:10 EET; 34min ago
  Process: 1009 ExecStartPre=/usr/bin/pvecm updatecerts --silent (code=exited, status=0/SUCCESS)
  Process: 1011 ExecStart=/usr/bin/pveproxy start (code=exited, status=0/SUCCESS)
 Main PID: 1013 (pveproxy)
    Tasks: 4 (limit: 4915)
   Memory: 127.7M
   CGroup: /system.slice/pveproxy.service
           ├─1013 pveproxy
           ├─1014 pveproxy worker
           ├─1015 pveproxy worker
           └─1016 pveproxy worker

Feb 07 23:18:08 proxmox systemd[1]: Starting PVE API Proxy Server...
Feb 07 23:18:10 proxmox pveproxy[1013]: starting server
Feb 07 23:18:10 proxmox pveproxy[1013]: starting 3 worker(s)
Feb 07 23:18:10 proxmox pveproxy[1013]: worker 1014 started
Feb 07 23:18:10 proxmox pveproxy[1013]: worker 1015 started
Feb 07 23:18:10 proxmox pveproxy[1013]: worker 1016 started
Feb 07 23:18:10 proxmox systemd[1]: Started PVE API Proxy Server.

I could connect to it using telnet 192.168.1.21 8006. Log displayed no errors. Runing by pveproxy -debug=1 start displayed nothing special in case of browser's page refreshing:

root@proxmox:~# pveproxy start -debug=1
9190: ACCEPT FH10 CONN1
9191: ACCEPT FH10 CONN1
close connection AnyEvent::Handle=HASH(0x560ee2f16cf0)
9190: CLOSE FH10 CONN0
close connection AnyEvent::Handle=HASH(0x560ee2f16cf0)
9191: CLOSE FH10 CONN0
9191: ACCEPT FH10 CONN1
close connection AnyEvent::Handle=HASH(0x560ee2f13ac0)
9191: CLOSE FH10 CONN0
9190: ACCEPT FH10 CONN1
close connection AnyEvent::Handle=HASH(0x560ee38bff60)
9190: CLOSE FH10 CONN0
9189: ACCEPT FH10 CONN1
close connection AnyEvent::Handle=HASH(0x560ee2f16cf0)

Answer 1

By creating the question, I was reviewing advice that did not work. I wanted to include them into the question to show what I've tried and what did not help.
Looking for that I've found an advice to check protocol. So I've discovered the cause of the issue.

I haven't used Proxmox very long time, so I forgot it should be accessible only through https://192.168.1.21:8006, not just by 192.168.1.21:8006. But after accessing it through https, Chrome remembered it and used https even if I typed http://...