Two days ago I installed Windows 10 1909, over 1903 (not clean install, just update). After that, search box in Explorer was broken for me too, but I finally SOLVED it by simply uninstalling WB and installing it again! Probably some files or settings, required for WB to function correctly, were "wiped out" by the 1909 installation and the re-installation put them back again. That also may happen for some other programs, after every bi-annual Win10 update.