Check to see if your anti-virus program has moved the Qt5Network.dll to quarantine folder. It happened to me today when I luanched WL 8.5.10 Avast reported Qt5Network.dll is a suspected virus and moved the .dll to quarantine folder. I had to restore the .dll and disable Avast in order to use WL 8.5.
Yes happened here as well with avast: I excluded it from scanning and uploaded the file to virustotal.com and it found nothing.
To restore from the Avast Virus Chest:
open avast user interface (Right click on icon in systray)
goto Statistics
goto component status
click on Items stored in Virus Chest
from there you can right click on the file and choose your option.