On initial launch after upgrading from previous beta, got this error:
The directory exists and is read-only. SYSTEM, the local Administrators group (of which I am a member) and my domain user all have Full Control on the directory.
Subsequent launching of Toad Beta did not raise the error.
I think what happened is that
a) when the beta started, it couldn't access SettingsLocations.ini
b) beta thought it had to create a settings folder
c) folder already existed and then error
Normally installing the beta while 16.3 is running should be ok. I think it was just unlucky timing that led to this. I'll look at the code to see if I can prevent it from happening in the future though.