I answered on another thread about a similar issue occuring in 4.19 & 4.20, but since that thread are not about 4.21.1 I’ll still update this thread.
I’ve tried updating my video driver (I have a Geforce GTX 1070) to the latest version (417.35) and even downgraded to 385.69 as suggested previously, but each time I change drivers, the issue goes away temporarily and then comebacks about an hour later. Once it’s back though, it’ll keep happening until I update my video drivers again.
My current specs are:
Unreal 4 version: 4.21.1
OS: Windows 10 64 bit
GPU: Geforce GTX 1070
CPU: Interl(R) Core™ i5-6600 CPU @ 3.30GHz
Ram: 15.92 GB