When you create an encrypted disk image in OS X, it defaults to creating the enclosed volume with restrictive permissions -- the owner (whoever mounted it) gets full access, while group members and others get no access. The web server normally runs as the _www system user, so it falls into the other/no access category. You can check this easily at the command line:
$ ls -l /Volumes
total 8
drwx------ 6 gordon staff 272 Sep 30 11:30 Encrypted Volume
lrwxr-xr-x 1 root admin 1 Sep 27 17:11 Macintosh HD -> /
The "drwx------" means it's a directory ("d"); the owner (gordon) is allowed read, write, and execute access ("rwx"), the group (staff) is allowed no access ("---"); and others are allowed no access ("---").
The simple solution is to open up the volume permissions to allow group and others read-only access with chmod:
$ chmod go+rX /Volumes/Encrypted\ Volume
$ ls -l /Volumes
total 8
drwxr-xr-x 6 gordon staff 272 Sep 30 11:30 Encrypted Volume
lrwxr-xr-x 1 root admin 1 Sep 27 17:11 Macintosh HD -> /
It's possible (but unlikely) that individual directories and files inside the volume also have restrictive permissions set, in which case you'll need to adjust them similarly.