Updated from rc14 to rc15 now getting this when OS tries to boot. Any ideas what's causing this?
 

Try redoing your flashdrive, but it may be a hardware issue/compatibility with the new 4.14 kernel.

I think I will just downgrade back to last working version and hope this gets fixed somehow.

 

If I understood correctly just copying files from previous folder to root on usb downgrades back to previous version. Is this right?

Just now, iletyz said:

Is this right?

Yes

Well... I did downgrading like mentioned above and now it goes to selection screen where you choose safe mode and memtest but it just counts down to zero and starts count down again. Can't select anything with keyboard either it just does nothing.

You likely have a corrupt flash drive, hence why I mentioned redoing it was a good option.