ntdll.dll is windows layer that interacts mostly between software and hardware. I would check your file system in this case. Seeing as the error is erratic and random it may be a precursor to a hardware failure of a hard drive or faulty memory.
Try the standard windows tools
SFC /scannow
and
dism /online /cleanup-image /restorehealth
to check your system files.
Try a malware scan try something like Malwarebytes for this.
Do a memory test via windows memory diagnostics mdsched.exe
Check if your hardware vendor has tools to check your hard drive health like samsung magician to see if the drive is in good health.
Verify your engine files from the epic launcher.
This behavior is far from normal.