It seems that after the latest package of sanctions, Zenhotels is no longer working properly in most European countries, though it still appears to function normally in the U.S. and several other regions for now.
If you're in Europe, you can try connecting through a VPN (set to a non-EU country), or use Triphotels (works when used in Spanish, French, Italian, or another language other than English)
You can find more details and alternatives in this article: https://russiable.com/ostrovok-best-alternative-booking/