What you described is a "tail pocket" a condition that is seen occasionally with any breed that commonly has a "srewtail" such as Boston Terriers, Bulldogs and Frenchy's. The accumulation of debris and moisture coupled with poor airflow over the area is what leads to problems. There are two approaches to dealing with this. The first is to keep the area cleaned using aseptic wipes such as Malacetic Wipes, the problem here is it needs to be cleaned multiple times per week forever. The second is to have the tail pocket obliterated by surgery. This eliminates the need for repeated cleaning but since the surgery is done in an area that is inflamed and infected there can be post surgical frustrations for you, your dog and the vet. I have had a few instances that required more than one surgery to eliminate the entire problem.