For issue with ‘UnsupportedClientVersion’ error returned by API, looks like basic authentication string used by binding is rejected by Honeywell API. If i use this from https://developer.honeywell.com/api-methods, then API responds correctly for me:
diff --git a/addons/binding/org.openhab.binding.evohome/src/main/java/org/openhab/binding/evohome/internal/api/EvohomeApiClient.java b/addons/binding/org.openhab.binding.evohome/src/main/java/org/openhab/binding/evohome/internal/api/EvohomeApiClient.java
index 6837eddbb..6f5978fd8 100644
--- a/addons/binding/org.openhab.binding.evohome/src/main/java/org/openhab/binding/evohome/internal/api/EvohomeApiClient.java
+++ b/addons/binding/org.openhab.binding.evohome/src/main/java/org/openhab/binding/evohome/internal/api/EvohomeApiClient.java
@@ -191,7 +191,8 @@ public class EvohomeApiClient {
+ "Connection=Keep-Alive";
Map<String, String> headers = new HashMap<>();
- String basicAuth = Base64.getEncoder().encodeToString((APPLICATION_ID + ":test").getBytes());
+ String basicAuth = "NGEyMzEwODktZDJiNi00MWJkLWE1ZWItMTZhMGE0MjJiOTk5OjFhMTVjZGI4LTQyZGUtNDA3Yi1hZGQwLTA1OWY5MmM1MzBjYg==";
headers.put("Authorization", "Basic " + basicAuth);
headers.put("Accept", "application/json, application/xml, text/json, text/x-json, text/javascript, text/xml");