This is probably due to Windows 10 ransomware detection. I had the same issue when starting the launcher (which loads the ffxivboot exe to install the new launcher) failed each time I tried.

Go to windows settings, search for "controlled folder access". Since you can't exclude the programs involved (they have not yet been downloaded), turn this feature off temporarily and restart the laucher, complete the update then quit.

Turn controlled folder access back on, and start the launcher again. When you have write issues (such as unable to update settings errors) then go back to this settings screen and add ffix related programs to the exclusion list.

HTH, /d