From what (external, wg0-connected?) machine with what IP address are you trying to access?
Or is your “not-work-statement” meant for your local machine where SC is running?
From your answer I infer that it should work, or at least that I didn’t make any obvious mistake.
I just tested it again, and actually locally it does work. Once I change the IP to 10.1.2.4 (2nd IP on wg0) in the Syncthing web interface, that is acknowledged positively by Syncthing. In the log I find:
Nov 15 09:56:18 linux syncthing[177102]: [7JOD4] INFO: GUI and API listening on 10.1.2.4:8384
Nov 15 09:56:18 linux syncthing[177102]: [7JOD4] INFO: Access the GUI via the following URL: http://10.1.2.4:8384/
Indeed I can connect to 10.1.2.4:8384, forwarded to my local machine. That instance of Syncthing is running on a server. And on that server, I get bombarded with messages output directly onto the terminal (SSH connection):
channel 5: open failed: connect failed: Connection refused
channel 6: open failed: connect failed: Connection refused
channel 7: open failed: connect failed: Connection refused
channel 8: open failed: connect failed: Connection refused
channel 9: open failed: connect failed: Connection refused
channel 10: open failed: connect failed: Connection refused
channel 5: open failed: connect failed: Connection refused
channel 5: open failed: connect failed: Connection refused
channel 6: open failed: connect failed: Connection refused
channel 7: open failed: connect failed: Connection refused
channel 8: open failed: connect failed: Connection refused
channel 9: open failed: connect failed: Connection refused
Connection via Wireguard does not work. In fact, also the other service that I have running on Wireguard stopped working, an HTTP file server on 10.1.2.3:8080. Somehow, binding Syncthing to wg0 breaks Wireguard.
I have no idea what is going on. It’s a weird issue.
Update (thinking aloud): These messages are from SSH. After binding Synchting’s web interface to a different IP, port forwarding from 127.0.1.1:8384 on the server doesn’t work anymore. (obviously)