When users try to open a lookup they get an error:

[red and white cross in a circle]


An error has occured

Try this action again. If this problem continues, check the Microsoft Dynamics CRM community ...

Google Chrome version 38

This error coincided with the release of Google Chrome 38 and appears to be the issue. Either use Internet Explorer, Firefox or go back to version 37.

Details here: www.crmanswers.net/2014/10/lookup-errors-with-google-chrome-38.html