svn copy svn://localhost/prs_med_srv/trunk svn://localhost/prs_med_srv/branches/v0.3 -m "Stable pre-release for demo."
svn: Path 'branches' not present

But, my directory tree is:

[root@waldorf prs_med_srv]# ls -l
total 40
drwxr-xr-x 3 root root 4096 Aug 23 12:32 branches
drwxr-xr-x 2 root root 4096 Aug 22 17:05 conf
drwxr-xr-x 2 root root 4096 Aug 22 17:03 dav
drwxr-sr-x 5 root root 4096 Aug 22 19:21 db
-r--r--r-- 1 root root    2 Aug 22 17:03 format
drwxr-xr-x 2 root root 4096 Aug 22 17:03 hooks
drwxr-xr-x 2 root root 4096 Aug 22 17:03 locks
-rw-r--r-- 1 root root  229 Aug 22 17:03 README.txt
drwxr-xr-x 2 root root 4096 Aug 22 17:08 tags
drwxr-xr-x 2 root root 4096 Aug 22 17:08 trunk