We came accross an issue where users could not download OAB.
While there were so many suggestions. (all of them are true that we need to check those).
But URLS, auth methods, and everything looks fine and if users stil get the said error message, please check the publishing settings of your OAB.
get-offlineaddressbook |select *web*
Set the values as following screenshot.
Special thanks to Stephen Wagner (https://www.stephenwagner.com/2017/11/06/offline-address-book-missing-exchange-2016/?unapproved=401750&moderation-hash=4e25eab310452ee58c8d0c39c7451772#comment-401750)