sharing files don't decrypt

Hello, I have a problem when sharing encrypted files, I've read all the documentation I've found on owncloud and I can not solve.

 I've been looking at are all shared keys and values ​​and the database table oc_filecache, files with the value 1 and the value encryptacion unencrypted_size appear.
I tried to regenerate users, key and reupload the files but there some kind of problem that can't resolve.

Can anyone give me a clue?

thanks a lot

Server configuration

Operating system: Debian 7
Web server: Apache/2.2.22
Database: MySQL Ver 14.14 Distrib 5.5.37
PHP version: PHP 5.4.4-14+deb7u10
ownCloud version:6.0.3

last lines in my owncloud.log:

{"app":"PHP","message":"json_encode(): Invalid UTF-8 sequence in argument at \/var\/www\/owncloud\/lib\/private\/log\/owncloud.php#73","level":3,"time":"2014-06-09T14:32:23+00:00"}
{"app":"Encryption library","message":null,"level":3,"time":"2014-06-09T14:32:23+00:00"}
{"app":"core","message":"no decode delegate for this image format `\/tmp\/fbca238fdde9b74441c82422e0cbf296pdf' @ error\/constitute.c\/ReadImage\/544","level":3,"time":"2014-06-09T14:32:23+00:00"}
{"app":"core","message":"Generating preview for \"ICA_Client_Object_Guide.pdf\" with \"OC\\Preview\\Unknown\"","level":0,"time":"2014-06-09T14:32:23+00:00"}
{"app":"core","message":"unable to open file `\/tmp\/magick-53iIJ5WA': No such file or directory @ error\/constitute.c\/ReadImage\/583","level":0,"time":"2014-06-09T14:32:23+00:00"}