Okay, i found a solution for my problem. Ich change the path in _wp_attachment_metadata by using a sql-statement. The values are saved as a serialized array, so i destroyed the form how it was stored, because i didn't correct the number of strings the new filename has. So WordPress trys to unserialize the corrupt serialized array, get a false, no data cabn be restored, so path, no edit, no size, so you get 0x0.
Little Example:
before:
s:4:"file";s:119:"/kunden/abcdef.de/webseiten/secure/www.abcdefghijklmnop.com/web/wp-content/uploads_de/asgdzenszajauaksddddsdsdsdsds.jpg"
after:
s:4:"file";s:119:"asgdzenszajauaksddddsdsdsdsds.jpg"
corrected:
s:4:"file";s:34:"asgdzenszajauaksddddsdsdsdsds.jpg"
It's better to use php and change the data with a script.
P.S.:
But before really check the right permissions. chmod 777 your testfile and the thumbs of them.