Problem updating Nextcloud Windows client

Nextcloud version (3.0.3):
Operating system and version (W10 Pro - latest):

Been using Nextcloud for over a year and updated many times but I can’t get the Windows 3.0.3 client update to install. I just get endless “Error opening file for writing” errors on "C:\Program Files(x86)\Nextcloud…
That folder IS writeable by me and it makes no difference if I run Nextcloud-3.0.3-setup.exe as Administrator - just gives the same errors.
Would appreciate any suggestions.