Thanks for your help, but it still doesn't work.
If I look in /var/log/messages I can see this:
apr 13 08:34:31 mail httpd: PHP Warning: Undefined index: use_vfs in /home/ht
tpd/html/horde/imp/compose.php on line 322
apr 13 08:34:49 mail httpd: PHP Warning: Undefined index: use_vfs in /home/ht
tpd/html/horde/imp/compose.php on line 322
apr 13 08:34:49 mail httpd: PHP Warning: fopen("/tmp/impattMDiNdY", "rb") - Fi
len eller katalogen finns inte in /home/httpd/html/horde/imp/compose.php on lin
e 329
apr 13 08:34:49 mail httpd: PHP Warning: stat failed for /tmp/impattMDiNdY (er
rno=2 - Filen eller katalogen finns inte) in /home/httpd/html/horde/imp/compose
.php on line 330
apr 13 08:34:49 mail httpd: PHP Warning: Supplied argument is not a valid File
-Handle resource in /home/httpd/html/horde/imp/compose.php on line 330
apr 13 08:34:49 mail httpd: PHP Warning: Supplied argument is not a valid File
-Handle resource in /home/httpd/html/horde/imp/compose.php on line 331
apr 13 08:34:49 mail httpd: PHP Warning: Unlink failed (Filen eller katalogen
finns inte) in /home/httpd/html/horde/imp/compose.php on line 332
the phrase "(Filen eller katalogen finns inte)" is swedish and means "The file wasn't found"