Managed metadata - bug with adding new terms?

I have a document library with managed metadata implemented. I have added Tags with terms. The term set is set to 'Open' and also to 'Allow Fill in Choices' to allow users to add their own terms if its not already there. However, when typing a new term, we get a message saying 'Please fix highlighted invalid terms before save.' 

This feature to allow users to type in their terms has worked for us previously on another Document Library and its now not working. Any ideas?