When you hover over the not found error, does it say “incorrect response code 1: not found”? That’s what it shows for me.
I think the other device is not connected to the relay.
You might have to add it as one of the Sync Protocol Listen Addresses on the other device (available in the Connections section). For example you can set the listen addresses to default,relay://bridgemode.bounceme.net:22067/?id=XWPAO25-SAO6M33-5I5WYJH-SMDM4A4-OLOI4B5-E2OC3S2-JZXUSDM-FUZQGQD to make the device constantly connect to the relay, but still keep the default listeners.
In Catfriend1’s Syncthing-Fork (or the discontinued official Syncthing Android app) you can set the listening addresses in Hamburger (3 bars) → Settings → Syncthing Options → Sync Protocol Listen Addresses.