Opera 12.18 - https connection with Avast 20.2.2401

I know is ancient browser and vulnerable but i need use it for one safe specific site where work best. But with avast installed it cannot connect to any https site because Avast use own certificate which opera dont know. Anytime i go to https site i get popup that certificate is not valid or creator of that certificate is not registered. My question is if is safe allow those certificates or not. Will be my connection still encrypted if i allow this? If i disable Web shiled i got no popup and everything work fine. Thank you for explanation.