Players may rarely experience this error when trying to update their SWL client: “An error occurred while updating SWL (app running)”
if you have this issue then it can be fixed by removing a temporary file that failed to be cleaned up correctly.
Navigate to the game’s installation folder. This is set by default to C:\Program Files (x86)\Steam\steamapps\common\Secret World Legends, though your installation location may vary if you have a different location set up for your games. Once you are in the installation folder locate the file named Lock2.txt and delete it. After doing so you should be able to run the game and update normally.