There are MANY different causes to this issue (especially troubleshooting client-side) but the main thing to check server side is on the IIS settings on the CAS servers. I had to make sure Basic and Windows auth was enabled on each of the following Virtual Directories: Autodiscover, EWS, OAB, and RPC.
The strange thing was the Exchange Server reset the RPC folder Windows Auth to “disabled” after a few minutes. I found many posts that seem to point to an Exchange Managment Shell command that will prevent this from happening, and it worked for me (so far it hasn’t reset since I ran the command). The command is this:
Get-OutlookAnywhere | Set-OutlookAnywhere -IISAuthenticationMethods:basic,ntlm
This is posted all over but it took a while to actually find references to this particular command would help prevent the RPC folder from resetting. Makes sense when you think about it, as even if you change the settings in IIS Exchange didn’t really know that so you also need to tell Exchange that these auth settings are allowed on the folder.