I upgraded to the latest version, but when I now visit http://www.jacktummers.nl/collectie/?sitemap or http://www.jacktummers.nl/collectie/index.php?sitemap I only get a blank page.

For now I have actived the default theme.

You need to create the sitemap files manually via the utility button on the admin overview page first. They are stored within /cache_html/sitemap as usual (divided by item type with an index sitemap file). The sitemap is not theme related.

On Zenphoto prior 1.4.1 they would have been created automatically on the fly. But that was changed because also could cause timeouts on your server easily. Especially on larger galleries.