MantisBT cannot seem to move files from a plugin temp folder to the uploads folder.

Here is the full error message incl. stacktrace:

APPLICATION ERROR #505

Uploaded file could not be moved to the file storage directory. Directory either does not exist or not writable to webserver.

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Full path: /var/www/mantisbt-1.2.15/core/file_api.php
Line: 711
Variable Value Type
p_bug_id 1795 integer
p_table bug string
p_title string
p_desc string
p_user_id 45 integer
p_date_added 1487744478 integer
p_skip_bug_update boolean
t_file_name CLPBRDohi7gd.png string
t_tmp_file /var/www/mantisbt-1.2.15/plugins/PastePicture/tmp/CLPBRDohi7gd string
t_file_size 150846 integer
t_max_file_size 10485760 integer
t_project_id 2 string
t_id 1795 integer
t_bug_id 0001795 string
t_file_path /var/www/mantisbt-1.2.15/uploads/findbuch/ string
t_file_hash 0001795 string
t_unique_name 3c2265221f8ee40b973736291b1a48de string
t_disk_file_name /var/www/mantisbt-1.2.15/uploads/findbuch/3c2265221f8ee40b973736291b1a48de string
t_method 1 integer

p_file
Variable Value Type
name CLPBRDohi7gd.png string
tmp_name /var/www/mantisbt-1.2.15/plugins/PastePicture/tmp/CLPBRDohi7gd string
type image/png string
error 0 integer
size 150846 integer

Filename Line Function Args
/var/www/mantisbt-1.2.15/core/file_api.php 711 - - trigger_error ( <string>'505', <integer>256 )
/var/www/mantisbt-1.2.15/plugins/PastePicture/pages/bug_file_add.php 105 - - file_add ( <integer>1795, <Array> { ['name'] => 'CLPBRDohi7gd.png', ['tmp_name'] => '/var/www/mantisbt-1.2.15/plugins/PastePicture/tmp/CLPBRDohi7gd', ['type'] => 'image/png', ['error'] => 0, ['size'] => 150846 }, <string>'bug' )
/var/www/mantisbt-1.2.15/plugin.php 57 - - include ( <string>'/var/www/mantisbt-1.2.15/plugins/PastePicture/pages/bug_file_add.php' )

Both the tmp and the upload folder have the correct chown & chmod settings. After uploading a file it gets stored in the tmp folder, where I can see it. But as soon as the file_api.php tries to move the file to the uploads folder it triggers the #505 error. I also tried it with 0777 for both the source and destination folder, this doesn't change anything unfortunately.
Everything works fine using the standard mantis fileupload (as in not PastePicture).