This issue may be connected with Redmine Favorite Projects Plugin from RedmineUP.
So, you have two ways for fix it:
Contact with RedmineUP and ask them about a new version that included this fix (Block REST API).
Download and install the last version of Redmine Shared API:
http://www.redmine.org/plugins/redmine_shared_api
https://github.com/anovitsky/redmine_shared_api