To the Avast users: most likely what's causing this issue for you is the AntiVirus' feature called "DeepScreen". That for me was the bane of many issues on multiple PC's, in fact I disable it as soon as I install avast.

I've just turned off my avast! Antivirus and GOG successfully installed DLC, thank you)

Congratulations for recycling a thread ! One gogbear didn't cry tonight :D

I'm not entirely sure but the update/installation disc access failures seemed to be remedied when I turned off "read-only" and numerous config files. So back those up, turn off read-only and let it run it's course and re-apply your configs.

Some people have also turned off their "Windows Event Log" in services.msc - a tweak related to numerous game performance.. This too can cause issues, particularly on your LAN connection, windows will disconnect online services like update, store and xbox on windows 10 if event log is off. If you find it fixes performance in some of your games, when you need to update or use windows services turn it back on.

Avast is just a hideous, useless product. Uninstall it.

Avast breaks so many things, causes blue screens. It's really not a viable product at all!

^ works. Damn Avast. Always trying to ruin our fun

I am with this error and I do not have Avast, only Windows Defender, and even disabling the error remains.
I'm using Windows 10 pro 64x, some other solution?

https://www.gog.com/forum/general/witcher_3_install_failed/post3