Material.cpp crash on game start

some of our players are getting this bug on game start which causes a crash. But not always. What could it be?

Assertion failed: !Material->IsDefaultMaterial() [File:D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\Materials\Material.cpp] [Line: 296]

One of the last things in the log is the
[2021.02.24-05.42.18:505][ 0]LogConfig: Applying CVar settings from Section [EffectsQuality@3] File […/…/…/CountryBallProject/Saved/Config/WindowsNoEditor/Scalability.ini]

Best regards