Whenever I try to edit (or preview an edit) of a page on wikipedia, I get the following error:

[WIdFwQpAAEYAAf10yWoAAACA] 2017-01-24 12:17:05: Fatal exception of type "ConfigException"

The page I was trying to edit was https://en.wikipedia.org/w/index.php?title=OPC_Unified_Architecture&action=edit

See https://www.dropbox.com/s/krfg8cp0kigqrb9/Screenshot%202017-01-24%2012.17.21.png?dl=0 for a screenshot.
Google Chrome, latest version.

Any suggestions welcome!