Because it also fails for non-admin users when not using IWA, this sounds like an issue with permissions on the share and/or server file system. I would start tracing those. To assign a library (and not update any tables) they should only need basic read and list folder contents access.
Once you have the non-IWA access resolved, and start looking at IWA access, if you you are using Windows 10 watch out for potential issues with constrained vs unconstrained delegation - for more info look at the Windows Defender Credential Guard section of Stuart Rogers' SAS Global Forum Paper SAS1878-2018: SAS® 9.4 on Microsoft Windows: Unleashing Kerberos on Apache Hadoop
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