Using the URL /search/ and /reverse/ (with slashes) is no longer supported. Please use URLs as given in the documentation.
Examples how to change the URL:
You use: https://nominatim.openstreetmap.org/search/?q=Berlin
Change to: https://nominatim.openstreetmap.org/search?q=Berlin
You use: https://nominatim.openstreetmap.org/search/US/Texas/Huston
Change to: https://nominatim.openstreetmap.org/search?q=Huston, Texas, US
See github issue #3134 for more details.