Cant figure out this intermitten problem DNS 500

I migrated to CI 3.1.5 and for the same function it works at time and other it gives DNS 500 error .  Compared 
for CI 2.2.x , it worked flawlessly.

I used files for session storage and I wonder this can be the cause but it doesnt make any sense if it does
and I wonder what else can contribute to this intermitten DNS 500 error.

Any opinion? 

Thanks