Can't save edited things

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I've installed both concrete5.7.5.13 and concrete5.8.0.3 on my 1und1 Ubuntu14.04 Virtual Server. Installation worked fine but if I want to save something I edited no change happens. I can cancel the saving process.

Nothing appears in the Network tab, only a Deprecated Message in the Browser Console

"Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, checkhttps://xhr.spec.whatwg.org/."...
Also no Errors in the Apache Log or the mysql log.

www-data is the owner of the files. Rights are 777 (hoping it would help).
max_execution_time = 600

I would be glad if someone can help! I'm running out of Ideas here.