Thanks to Michael https://www.opensource-socialnetwork.org/u/zetman

I found the error! The system-info tool is great and really helpful.
The "html"-dir wasnt owned by the webserver.
Chowned it to www-data and everything worked like a charme.
Thank you very much.

Regards,

Why is the system-info tool not included in the ossn archive? This would help many users and save you from annoying questions like this one. ;) Just my fifty cents...

This error occurred when isCacheWriteable return false means installer tried to write a folder called cache and if it failed it return error.

Follow case A https://www.opensource-socialnetwork.org/wiki/view/1101/how-to-report-a-bug-error

Nope. No cache folder. Its a clean install. I had no OSSN running on that server before.

Can you check if there is already cache folder exists? in root?

Hi, Arsalan!
You can find all the folder and file permissions here: Link to txt-file
And here is the screenshot of the installer: screenshot of error

Thank you very much.

Please provide us the screenshot if possible.

Edit:
All files are 644 and all directories, including the ossn_data-dir, are 755.