Can't add or update a taxonomy term Something has happened on my site where I am unable to add taxonomy terms or edit existing ones. 

If I go to add a taxonomy term or edit an existing one the form comes up, I made an changes (i.e. add some text in the description field) and then click save and it goes back to the Taxonomy list  but nothing changed. When I go back into edit the taxonomy term the previous change is gone. 

The same thing happens when I try to add a term through /admin/structure/taxonomy/name_of_vocabulary/add form- it saves, but then the new term doesn't show up. 

Any ideas?
Solved by upgrading to the most recent version of Meta tags module. 

Thank you to Patrick Kenney for pointing me in the right direction.