I got similar error:
On first attempt (update to NC 3.0.3 - Windows 10), while not stopping the client before installation and not using an admin account, there was: “Error opening file for writing: C:\Program Files\Nextcloud\Qt5Core.dll”.
On second attempt “run as asdmin” while nc stopped, there was: “Error opening file for writing: C:\Program Files\Nectcloud\shellext\NCContextMenu.dll”.
Attempt to select “uninstall nextcloud” showed “an uninstall already in progress”.
Thus, I used following workaround:
I uninstalled from Windows menu “uninstall or install a program” (successful).
Then reinstalled (“run as admin”) successful without any problem, BUT settings were lost. Just had to re-logon (in nc sync client) and select the folders to be synced again. All fine.
Hope this helps.
Good luck!