"MU Legends" does not start (874240)

Compatibility Report

• Name of the game with compatibility issues: MU Legends
• Steam AppID of the game: 874240

System Information

• GPU: Radeon RX 550/550X
• Driver/LLVM version: OpenGL version string: 4.5 (Compatibility Profile) Mesa 18.3.2
• Kernel version: 4.20.7-1
• Link to full system information report https://gist.github.com/ensonic/58ea92c850a41c499d3fa9914476ec19
• Proton version: 3.16 beta

I confirm:

• that I haven't found an existing compatibility report for this game.
• that I have checked whether there are updates for my system available.

https://gist.github.com/ensonic/3f5461e62c8fabdd7c18374fd76e1ede

Symptoms

The webzen launcher appears and prints (in broken english)

Oh nose!
An error has occurred. Please restart Steam and try again later.
Apologies for the inconvenience.
If the issue persists, please contact customer support for more information. 

The might be related to https://github.com/ValveSoftware/Proton/issues/162. The launcher looks like as if it wants to download additional files (or check for updates).

Reproduction

Run the game.

Start by reviewing the attached Proton log at $HOME/steam-874240.log and the related Proton issue 162, then reproduce the failure by running MU Legends with Proton 3.16 beta. Done means the Webzen launcher no longer shows the reported error and the game starts.