Relay always damaged in switching circuit
You are welding the contacts of the relay by the surge current,  probably caused by capacitance in the power supply front end.
One approach would be to use a physically bigger relay with a better surge current rating. Probably a TV-5 rated relay would be acceptable, though the rating is UL/CSA so it's limited to 120VAC, so make sure the relay itself is rated for your mains (250VAC etc.). See this from Omron's data.