No JSON object could be decoded u1-darwin

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Environment: OS 10.6.8/ 2.1GHz Intel Core 2 Duo/ § GB 667 MHz DDR2 SDRAM, no proxy/firewall or whatsoever

Logs: see attachement

Problem:
I can't use the UbuntuOne-App on my MacBook anymore. Whenever I start the App the symbol in the dock starts to jump and a window appears in front of the main-window, which says:

"Es tut uns Leid, es ist ein Fehler aufgetreten und Ubuntu One muss beendet werden." (I'm Using Ubuntu One in German.)
-> In English: "We're Sorry, an error occured and Ubuntu One has to be closed."
When I click on "Show Details" it says:
"ValueError
No JSON object could be decoded"

I installed Ubuntu One at the end of November 2013 and it worked fine until about one week ago.
I didn't change anything about my OS or my account.

I already tried deleting and installing it again. The error stays the same.
The version of Java on my Mac is Java SE 6, Version 1.6.0_65_b14-462 (CPU-Type 32-Bit).