There is a bug in asking for back-fill via QLink. We're in the process of improving the base code for QLink and should have a 3.0 version due out by end of Q1 next year that will be more robust and more stable.
In the interim, the only way to counter the bug is to try a different request types since this is such a random bug. For example, if you're looking for 100 5-min bars, try 101 5-min bars. If that doesn't work, try 102 5-min bars, etc. Certainly not ideal but that's how some customers are getting around the issue for now.
Sorry for the inconvenience. The new version will be much improved.
Thanks.