I used the quick upload, but it doesn't works correct.

I work like this:
1. I logged in into the blog on the front end.
2. I click onto "Edit" under the post.
3. Under the post in the backend, I click onto "Link existend files" ("Existierende Dateien verlinken" in german).
4. I chose the PDF - 28 kb - on my desktop.
5. I click to upload it.
6. I got this error message:
20150613-jazzclub-sommerfest.pdf
Notice: Trying to get property of non-object in /home/dirname-of-the-server/blogs/htsrv/quick_upload.php on line 245
{"success":{"text":"V2lyIGvDtm5uZW4gZGVuIE9yZG5lciA8Yj4vaG9tZS9lZG5vbmcvcHVibGljX2h0bWwvYmxvZ3MvbWVkaWEvPC9iPiBuaWNodCDDtmZmbmVuLiBQSFAgYmVuw7Z0aWd0IGbDvHIgZGllc2VuIE9yZG5lciBBdXNmw7xocnVuZ3NiZXJlY2h0aWd1bmdlbi4=","status":"error","specialchars":0}}

The file name is only 20150613-jazzclub-sommerfest.pdf - no special chars in it.
The system here: Kubuntu 14.04, newest Firefox.
In the PDF there is only text, nothing more.