Popcorn Dave - While the permissions you suggest (755) would certainly be preferrable, that would not cause the 500 error. 755 indicates read-write-execute for the owner(7) and read-execute for the group(5) and the world(5). 775 indicates read-write-execute for the owner(7) and the group(7) and read-execute for the world(5). You can find a more detailed explanation of UNIX permissions here.

-- vek --