When running Substance Painter on Ubuntu via Steam, some buttons that should open an URL (web page) might not work.
A workaround to this issue is to enter a command in the properties of your application to enable these buttons back again.

Setup of the command

1 - Right-click on the application in Steam and choose "properties" 

2 - Click on "Set Launch Option..."

3 - Enter the following command in the text field, then press OK and close the properties window.

LD_LIBRARY_PATH="$SYSTEM_LD_LIBRARY_PATH" %command%