I get an error in Adwords saying that "A negative keyword is preventing your ad from showing". Why?

Negative keywords are used by AdNabu to eliminate internal competition. This ensures that a search which can be triggered by exact matchtype is only triggered by exact and not by phrase or broad. You will encounter this error in phrase and broad keywords and the reason is that there is an exact keyword which should ideally trigger the ad.

This is by design and no changes needs to be done by you.