I already saw similar questions and I also face this problem in my mobile project.
I think its because UMG blur not supported on mobile devices. Maybe it will work on some hi-end/metal render, but I not checked. So in my project I just disabled this and use simple mid-black translucent background.
#if PLATFORM_ANDROID
// This feature has not been tested on es2 and will likely not work so we force low quality fallback mode
static int32 bForceLowQualityBrushFallback = 1;
#else
static int32 bForceLowQualityBrushFallback = 0;
#endif
If you want to enable background blur on android device(vulkan device?), add option to DefaultDeviceProfiels.ini
or use commandline.
A test doing it on Nexus 5x failed. Either it was wrong to put the file DefaultDeviceProfiles.ini in myproject/config or it is not supported on older devices.