In case you receive the error message
Unable to find a sitemap to generate
when trying to generate a Google Sitemap for an existing entry in Magento make sure to check that the sitemap ID generated is set correctly, i.e. not equal to 0 (zero).
In this particular Magento 1.9.1.0 setup the ID for the first sitemap wrongly was set to 0 on generation via Catalog / Google Sitemap, thus breaking the sitemap generation process through the Generate option: 
Correct Sitemap ID in Database
Manually setting the sitemap ID in the database to 1 for the first sitemap solved this problem:
Still, it’s strange that no error message was displayed when creating the first sitemap entry through Catalog / Google Sitemap.
Has anybody else experienced this behavior?