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. (The debug.log didn't show any errors, but I can share it if you insist)
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. No software blocks port 8333.
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
Edit: Just stopped again:
2024-01-13T19:37:11Z UpdateTip: new best=0000000000000000000240f175b8c1d2be32786af8fcad523989f39733c8ec91 height=825246 version=0x20400000 log2_work=94.658582 tx=950438404 date='2024-01-11T05:27:27Z' progress=0.999320 cache=217.7MiB(1461058txo)
2024-01-13T20:08:41Z Potential stale tip detected, will try using extra outbound peer (last tip update: 1890 seconds ago)
2024-01-13T20:19:11Z Potential stale tip detected, will try using extra outbound peer (last tip update: 2520 seconds ago)
2024-01-13T20:29:41Z Potential stale tip detected, will try using extra outbound peer (last tip update: 3150 seconds ago)
2024-01-13T20:40:11Z Potential stale tip detected, will try using extra outbound peer (last tip update: 3780 seconds ago)
2024-01-13T20:50:41Z Potential stale tip detected, will try using extra outbound peer (last tip update: 4410 seconds ago)
$ bitcoin-cli -getinfo
Chain: main
Blocks: 825246
Headers: 825666
Verification progress: 99.9304%
Difficulty: 73197634206448.34
Network: in 0, out 0, total 0
Version: 230000
Time offset (s): -1
Proxies: 127.0.0.1:9050 (ipv4, ipv6, onion, cjdns)
Min tx relay fee rate (BTC/kvB): 0.00001000