I got this same error after updating to EPiServer.CMS.UI 11.14.0. It is preventing the publish menu from dropping down correctly
When I downgraded to EPiServer.CMS.UI 11.13.2, the drowndown menu worked fine.
Hi I got the same issue with package 11.16...
Also seeing this on 11.16. Am going to try upgrading Epi and see if that helps.
Did you ever find a resolution?
Do you have Episerver.Marketing.Testing package installed in your solution? if yes, which version is currently installed?
There is a known bug on version 2.5.4