Safari cannot establish a secure connection with icloud.com and idmsa.apple.com

Safari cannot establish a secure connection with icloud.com and idmsa.apple.com but can connect with everything else. It will connect to apple.com, but not to appleid.apple.com. Booting with reset computer connects to icloud.com. Does not connect in safe mode or Shift Safari. Spent several hours on chat with tech support without success. Reinstalled High Sierra and updates. Tested with new userid. Everything works except Safari won't establish a connection on any Apple site requiring a login. Installed the Avast browser which has allowed me to connect and post this question (along with connecting to icloud.com). This is an older computer running High Sierra. What is wrong with the Apple browser?