Add a print_r($ptr['d']) to the same location as before an report back 
on the results.

And what that means?

Looks like $ptr['d'] is containing a PEAR_Error then.

var_dump($key) gives string(4) "msgl"

This error message makes no sense.  Everywhere we call getCache() in 
IMP, the 3rd parameter ($key) is *always* a static string.  You will 
need to add a print_r($key) or var_dump($key) in IMAP_Cache right 
above line 80 to figure out what $key is.

I tested with rc2 and 14.02.2008 nightly build

Tested with file and eaccelerator cache

I'm not entirely sure I follow. Are you actually using HEAD? If so 
when was it updated?

If horde cache is configured then IMP to not show mails gives error:

Cannot use object of type PEAR_Error as array in 
/srv/www/htdocs/horde/lib/Horde/IMAP/Cache.php on line 80