Do a hard reset. happened to me and this fixed it note: Please be aware that Apple may screw you if you try to un-pair and re-pair the watch or update the watch. see Why won’t my watch pair after screen replacement?
yup, same here. watch series 2 4.2.something firmware, ios 11.4 iphone firmware.. seems like thease cases will begin to pop up. Unfortunatley I have had no luck in figuring this out. Some say, that by flashing by ibus s2 could fix the issue but unfortunatley there are no ISPW firmwares for apple watch s2 anymore that are being signed by apple.. so not an option here... However: IF you attach the original screen, pair the watch and then switch to the new screen the watch will work ( more or less... it won't update to 4.3.x complainig about update that could not be verified and of course if you erease you won't be able to pair again...) Still hoping this is a bug somewhere... and either ios 12 or watchos 5 will fix it and I'm being positive that apple does not want to screw us all.. Anyway..guys.. would be helpfull if you could try to pair with an Iphone running FW 11.3.xy