Custom CRM/Mailing/Info.php causes WSOD when installing extensions
There are a few files across the system that cannot be over-ridden via the custom php directory. This is primarily because these files are included during the bootstrap process.

I dont think anyone has documented which files or tried to minimize the number of files that can be overridden. I suspect all component Info.php files fall in  this bucket and hence you get the above error

Curious as to what custom code you are adding to the Info.php file