Hm. Weird. I added debug=tor to the configuration file, rebooted the device, and it now works. Tor runs fine, Bitcoin Core is syncing. I literally did not change anything else, and that's the bad part, because I don't know what was the fault so I can fix it in the future.
Any new antivirus or firewall software or updates which could be blocking ports, etc?
No, nothing. I had followed Raspibolt's installation tutorial quite a long time ago.
It looks like the DNS server you are using isn't returning any responses for the DNS seeds. Have you specified any particular DNS server? What response do you get if you get DNS records from the DNS seeds with dig?
Oh, I must have screwed it up with DNS. Not entirely sure, but I think I had changed something around that last year. Here's dig's output:
$ dig
; <<>> DiG 9.16.42-Debian <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56056
;; flags: qr rd ra; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1220
; COOKIE: 3bce0aa3567b5d1249fe618865a2decae8ab20eb1454add7 (good)
;; QUESTION SECTION:
;. IN NS
;; ANSWER SECTION:
. 440064 IN NS i.root-servers.net.
. 440064 IN NS m.root-servers.net.
. 440064 IN NS a.root-servers.net.
. 440064 IN NS d.root-servers.net.
. 440064 IN NS f.root-servers.net.
. 440064 IN NS c.root-servers.net.
. 440064 IN NS l.root-servers.net.
. 440064 IN NS j.root-servers.net.
. 440064 IN NS k.root-servers.net.
. 440064 IN NS h.root-servers.net.
. 440064 IN NS e.root-servers.net.
. 440064 IN NS g.root-servers.net.
. 440064 IN NS b.root-servers.net.
;; Query time: 11 msec
;; SERVER: 192.168.2.1#53(192.168.2.1)
;; WHEN: Sat Jan 13 21:04:42 EET 2024
;; MSG SIZE rcvd: 267