Group Icons cant be uploaded

Hello,

i have an Debian-Server with TeamSpeak 3.0.11.4 on it. When i upload an Icon i can see it, but my Friend gets the Message:

Code:

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The Icon of the Servergroup "XXX" was not found

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When i try to Upload Group-Icons i get this Error in my Client Log:

Code:

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07.01.2016 13:09:50        ClientUI        Error        Error getting transfer sender state

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The Serverlog is clean and i already find out that the Port 30033 has to be open (it is).
Also all other Port used for Teamspeak are Open.

An Iconfile is not generated. The Server has got all needed Permissions.

Does someone has an Idea how to fix this strange Problem?

Best Regards,
Yannek

Group Icons cant be uploaded