Steps to Reproduce
Set the user agent string to: "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_1) AppleWebKit/602.2.7 (KHTML, like Gecko) Version/10.0.1 Safari/602.2.7".
Log in to the instance.
Note that UI11 appears.
The only workaround for the issue is to use a different browser, or to upgrade to a patch release that contains the fix. To receive notifications when more information is available, subscribe to this Known Error article by clicking the Subscribe button at the top right of the article. If you are able to upgrade, review the Fixed In field to see which versions have a permanent fix.
Related Problem: PRB713397