When launching the latest version of the itch.io app on Windows 10 1803, I get the error message:
"There was a problem during setup: sanity check for butler failed with code 3221226505, out = , err ="
This seems to be caused by an incompatibility with "%APPDATA%\itch\bin\butler.exe" and Windows' Control Flow Guard security feature.
Creating an exception in Windows Defender's Exploit Protection settings for this process works as a temporary fix.