The problem is that you are joining SD to two tables. Explain this set of predicates first, as it doesn't seem to make any sense:

Code:
   D.APP_REF_NR       =   APPL.APP_REF_NR      AND
   SD.APP_REF_NR      =   APPL.APP_REF_NR      AND
   SD.DISABILITY_ID   =   D.DISABILITY_ID      AND
I also noticed that you are not using the GAP_SEG_DISABILITY_TYP table in the resultset, so I'm not sure why it is included.