Have you configure in Lightroom the memory cache?
WWhat OS? How much installed RAM? How much free memory is left after usage by other apps you have open and those that opened automatically at start up?
Normally when Lightroom says "not enough memory", that's exactly what the problem is, you don't have enough memory.
A partial solution would be to make sure no other applications are running when you do this export.
The real solution is to make sure your computer has both 8GB RAM (or more) and a 64 bit operating system. One of these by itself without the other will not improve the situation.
Thanks for your comments and tips. The problem doesn't exist any longer, because I have updated Lightroom to version 5.5 and now there is no problem any more. Thanks once again