Well, apologies for the misdirection, but it turns out our TaxonomyManager can work only with one provider. You can inherit from the default OpenAccessTaxonomyProvider and reregister it. Then it should work with our default taxonomies and with your custom taxonomies.
Sorry again for the misdirection.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items