When you click on a search, while the WebClient is opened, this opens up, but the loading bar does not stop to run at the top of the screen. A search can indeed be entered, but the "Send" button cannot be executed, so it is not possible to search, if you now press F5 the loading bar disappears and you can perform a search once. If you want to do a search again directly a search does not start until pressing F5. If you now change the display mode, the viewer can be opened in a separate window, the search works, but the viewer only works for the first Document and its first page.
If you have a Windows system with a 64-bit version, please open Internet Explorer as a 64-bit version. This behavior can be triggered by different addons, that are installed.