Hi,
Some of our field staff are also experiencing this 'untrusted host error' when using the latest version of collector on Android devices.
The error message is specifically "Untrusted Host https://services2.arcgis.com/......... This server could not prove itself; its security certificate is not trusted by your OS. Would you like to continue anyway?
The current work around is to have users log out of Collector and log back in.
I've just suggested that field staff uninstall / reinstall collector as per some of the suggestions above to see if this resolves the issue
ESRI is there an official solution for this problem yet?
Thanks,
Clinton