ProxyURL is not designed to work with httpAuthNtlm, don't use it and there is no problem.   httpAuthNtlm uses Windows credentials, that is the whole point of it.      Mind another ICS user made changes that might be the cause of your problems:   Apr 09, 2019 V8.61 OAS : Improved NTLM authentication by adding Single Sign On with NTLM Session on Windows Domain to get credentials without needing them specified here.
  Angus