Firstly, you will find a screenshot of how Web Display looks when it first starts up in Appendix D of the User Manual.
As it's a web server the display is the same whether you access it directly from a browser, or via the MSFS Toolbar Add-On.
Coming back to the problem that you are having, your latest screenshots clearly show the cause:
Web Display tries to open a log file in it's home folder and this is being trapped by Windows UAC as it regards it as a protected folder.
It is possible that this issue has been caused by changes introduced by a Windows upgrade.
Did you install PF3 as 'Administrator'? Installing as Administrator should avoid this problem.
Open the FS_Addons folder in file Explorer, right-click on the PF3 folder and select the Security tab; click on the Advanced button - who is the Owner?
Perhaps you could upload a screenshot of that window.
In the meantime I will see if I am able to reproduce the problem that you are having.
Thanks,
Martin