sitemap not writable error after sitemap generation

[mike94]

Hello,
I generated a sitemap for my site, and all went well.
The new sitemap was uploaded and is correct.
However, when I go to the 'configuration tab' I get error messages now. ( sitemap is not writable )
See attachment.
These files do not exist in the directory, my sitemaps are named differently.
( sitemap_nl.xml instead of sitemap.xml )
How do I get rid of those errors ?

[XML-Sitemaps Support]

Hello,

make sure that data/ folder is writable.

[mike94]

The permission is 777

[XML-Sitemaps Support]

Please check that that "HTML sitemap filename" setting is defined correctly.

[mike94]

The "Create html site map for your visitors" was unchecked, so I don't have a sitemap.html file.

If I enable it, the error stays the same.

It was unchecked, but a path to the sitemap was there.
If I remove the path to the sitemap, and uncheck it again, I get

------------------
An error occured

Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:
Sitemap file is not writable:

[XML-Sitemaps Support]

There must be a full filename in that setting.