Dear Marco,
as explained in the Users Manual chapter 9.1 "Unable to Write Entry" and in FAQ trouble shooting
/faq#sec-14 this error message means that you have a permission problem somewhere.
This problem may occurs when the users that has installed Joomla the first time is not the same than the one that perform the update.
So check the owner of the files and directories and their permissions.
You have also tried to activate the FTP Layer and the parameters seems incorrect.
This is an excellent idea as this generally allow to bypass the permission problems.
Remark:
- the Joomla FTP Layer is only available for Unix servers.
- At my knowledge, Joomla does not support the FTP Layer for Windows servers.
If you are using a Unix server, you can use the same parameters than the one you are using to connect your server by using an external FTP tool.
In your case, perhaps the FTP port 21 is not available for security reason.
I hope this will help you