Please try this:
- remove all mapping for Alt+Enter shortcut here Tools | Options | Environment | Keyboard (to find out mapped actions, press Alt+Enter in "Press shortcut keys" field and look at the list in "Shortcut currently used by" dropdown);
After removing:
- Type ReSharper_AltEnter in "Show command containing" field;
- Select "Text Editor" in "Use new shortcut in" dropdown;
- Press Alt+Enter in "Press shortcut keys" field | Assign;
- Select "XAML UI Designer" in "Use new shortcut in" dropdown;
- Press Alt+Enter in "Press shortcut keys" field | Assign;