Probably because 4.8 is not included in the .net cu? Here you can find dotnetfx48: DotnetFx48 https://forums.mydigitallife.net/threads/windows-10-hotfix-repository.57050/page-388#post-1515194 DotnetFx48 LP's https://forums.mydigitallife.net/threads/windows-10-hotfix-repository.57050/page-388#post-1515291
Windows 10 LTSB 2015 (x86/x64) and Windows 10 LTSB 2016 (x86) requires a special patch or fix made by mdl member abbodi1406 for integrating the CU's. You can get the fixes from abbodi1406 which needs to be applied before integrating the CU. The patch for Windows 10 LTSB 2016 (x86) is also avialable in my Toolkit which is based on abbodi1406 patch. The patch for Windows 10 LTSB 2015 (x86/x64) will be avialble in the next version of my Toolkit.
Could be that dreaded "USB storage device" being connected that the new 1903 doesn't like. I'd disconnect all USB stuff, reboot and try again. Could leave the mouse on if you need it and don't have a ps2 one handy.