I was trying to install Epic Games on my Lutris installation on my old laptop and ran into the following error message:
Exit with return code 256
(This appears to be a common issue and is not necessarily limited to Epic Games.)
I actually removed my default Ubuntu package which was a few minor versions behind the official packages. I then installed the latest version of Lutris directly using a .deb file from their website.
This still did not seem to fix the issue although (from memory) the installation seemed to move a little further than previously. I googled again and was told that removing a hidden .wine folder in my user profile directory would reset lutris and fix the issue. But nope.
Now, the thing is that the code 256 was displayed in a small terminal window which had other information related to the error. The most recent was a warning that andale32.exe could not be downloaded and could not be found in a specific directory (.cache/winetricks/corefonts) in my home folder. A URL (https://mirrors.kernel.org/gentoo/distfiles/) was also provided. I found that I could download it manually myself. So I moved the downloaded andale32.exe file to the aforementioned corefonts directory in my home folder and reran the Epic Games installation within Lutris. Unsurprisingly, the installation again failed but it was not complaining about a different font (Arial) rather than Andale. So I decided to download all the font files from the mirrors.kernel.org site and place them in the corefonts directory.
When I reran the installation, it was successful!
I hope this helps someone out there. Good luck.