Seeing the same error in the Pi-Hole Diagnostics, once yesterday and once today on a Pi 3+ running Pi-Hole v6 (updated from v5) and Unbound locally on the Pi 3B+. And saw the same error yesterday on a Pi Zero W also running Unbound locally that likewise was updated from Pi-Hole v5 to v6. From the Pi 3B+'s FTL.log file:
2025-02-20 15:09:20.302 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 15:24:45.742 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 15:25:15.606 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Connection prematurely closed by remote server)
2025-02-20 15:26:32.622 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 15:47:19.982 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 15:48:29.742 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 16:11:47.502 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
2025-02-20 16:14:44.142 WARNING Connection error (127.0.0.1#5335): TCP connection failed while receiving payload length from upstream (Resource temporarily unavailable)
Checked the syslog file on the Pi 3B+ and don't see any error offhand around that same time.
Edit to add: Nothing has changed on the Raspberry Pi's other than updating Pi-Hole. Did not see, as far as I can remember, these kinds of errors in Pi-Hole v5 on either Pi device.