When viewing a Homepage, you click on the Add Content button to load additional reports/lists/etc. onto the current Homepage. However, the 'Add here' buttons at the bottom of the new window are not visible.
For example, there could be a UI action that is configured as a 'List Context Menu' which will cause it to render any time a list for the determined table is requested (assuming the current user meets the 'Condition' criteria). If you have a List Report on the Homepage, then it is expected behavior for the UI Actions to be loaded with it. Another example could be a global UI Script causing the same issue
Use the console error to track down the source of the problem and correct the issue.