Hi @lshema
Thank you very much, that was helpful. I thought the error message is about a HTML header, but it seems to be something WordPress related.
I was now able to reproduce the error and am going to work on a fix.
The error just occurs if you install the Sitemap Generator from within WordPress and try to activate it directly.
If you switch to the “Installed plugins” page and activate it there, it works fine. This can be used as workaround till the issue is fixed.
Best regards
Marco