Does potassium nitrate solution degrade? I have created a solution of $\ce{KNO3}$ and table sugar in order to create fuse from hemp rope. I wet the rope in the solution and let it dry - I repeat this process several times.

It works and produces very slowly burning rope which is easy to set off. The problem is that after 24 hours, my solution started to stink. Something between garlic, onion and vinegar. I have a bad feeling about that.

Does my solution degrade, or is this stink caused by some impurities (the solution isn't very pure)?
As mentioned in the comments, a likely culprit is microbial contamination.  In a $\ce{KNO3}$ solution, you are providing plenty of water, a nitrogen source (nitrate), and even a high-energy, respiratory means for growth (nitrate respiration).

Living things also need (i) food, (ii) trace amounts of sulfur, (iii) and even smaller trace amounts of phosphorus.  But if you are soaking hemp in this solution, enough leachable S and P-containing impurities are probably extracted into the solution to provide these.  (Or also low levels of S and P may already be present in your $\ce{KNO3}$, or water.)  The food could be organic compounds leached from the hemp.  Or (less likely) the microbial contaminants might be capable of photosynthesis, in which case their food is atmospheric $\ce{CO2}%edit$.