I personally have never heard of the Knight LK-98. Of course that does not mean it does not exhist. I have a Knight LK-93 II which is the older model. I will guess that your rifle is like a Wolverine LK-93 that uses the red 209 primer holder.

What you need to do is break that rifle down and clean it. Take the breech plug out, and clean it free of fowling. Be sure and clean the barrel ASAP. Although APP is a forgiving powder, all of these powders will rust your rifle sooner or later. So clean them ASAP after shooting.

Take the barrel out of the stock by removing the locking lug screw on the bottom of the rifle. Then pull the back part of the rifle out. If you have a manual it will give you detail on how to break the rifle down. But you do need to get that thing apart, clean it will, re-grease the breech plug and then assemble the rifle clean with a nice coat of oil in the barrel. Then before you shoot, simply run a patch with alcohol down the barrel to remove the oil, a dry patch or two, pop a couple 209 primers to clear the breech and then load as normal.