So in my case it turned out the problem was related to Horde's temporary directory. The fix Parallels provided is:
1. Check /usr/share/psa-horde/config/conf.php for $conf['tmpdir'] = '/tmp';
2. ls -la /tmp find the folder '.horde'
3. Permissions on /tmp/.horde were apache.apache
4. chown -R...
I just wanted to add that I'm also experiencing this problem on a freshly updated Plesk 11.5.30 box. Once the update went through, Horde would no longer send attachments correctly, either forwarding or in a fresh email.
Anyone at Parallels care to comment or provide a patch?
Upgraded to 10.3.1 and the "Make a Copy" function is still not working. It creates a new database, but doesn't create a user and doesn't import any data or table structure.
Any word on when this might be fixed?
No, you're quite right. I'm having the same problem (albiet on Windoze, with 10.3.1) and none of the --reconfigure-vhost or --remove-vhost commands help at all.
Very frustrating. So far Plesk 10 for me ranks as the worst Plesk release I've ever used.