A customer was unable to save or change any managed metadata fields within a specific site collection on a SharePoint 2010 environment. The customer did not get any error message or warning. The situation was as follows.
1. The user edited an item and changed a managed metadata field.
2. The user saved the changed item.
3. No error message was generated and the managed metadata field was not changed.
SharePoint uses a hidden list called the TaxonomyHiddenlist to cache all the available taxonomy terms for a specific site collection. By default NT AUTHORITYAuthenticated Users has read permissions to this list. In the customers case the default permissions where somehow removed or never set. The issue was resolved by granting NT AUTHORITYAuthenticated Users read access to the TaxonomyHiddenlist.
Taxonomy Hidden list details
1. The TaxonomyHiddenList is located on the root site of every site collection. The URL is http://[site collectionURL]/lists/TaxonomyHiddenList.
2. The TaxonomyHiddenList contains all the terms and keywords used withing the current site collection.
3. SharePoint uses the list as a cache, by caching the terms in the hidden list there is no need to query the managed metadata service all the time.
4. A taxonomy term is cached when a term has been used in a list. The following values are stored for every cached term.