diff --git a/src/ScadaLink.Commons/Serialization/OpcUaEndpointConfigSerializer.cs b/src/ScadaLink.Commons/Serialization/OpcUaEndpointConfigSerializer.cs
index 79f6ff3..60b305d 100644
--- a/src/ScadaLink.Commons/Serialization/OpcUaEndpointConfigSerializer.cs
+++ b/src/ScadaLink.Commons/Serialization/OpcUaEndpointConfigSerializer.cs
@@ -118,8 +118,27 @@ public static class OpcUaEndpointConfigSerializer
 
     private static OpcUaEndpointConfig LoadLegacy(string json)
     {
-        var dict = JsonSerializer.Deserialize<Dictionary<string, string>>(json)
-                   ?? new Dictionary<string, string>();
+        using var doc = JsonDocument.Parse(json);
+        if (doc.RootElement.ValueKind != JsonValueKind.Object)
+            throw new JsonException("Legacy JSON must be a flat object.");
+
+        var dict = new Dictionary<string, string>();
+        foreach (var prop in doc.RootElement.EnumerateObject())
+        {
+            // JsonElement.ToString() returns the raw value for primitives:
+            // "1000" for numbers, "true"/"false" for booleans, the string content
+            // for strings. Nested objects/arrays serialize to their JSON; we don't
+            // expect those in the legacy flat-dict shape, but FromFlatDict will
+            // simply ignore unknown keys.
+            dict[prop.Name] = prop.Value.ValueKind switch
+            {
+                JsonValueKind.String => prop.Value.GetString() ?? "",
+                JsonValueKind.Number => prop.Value.GetRawText(),
+                JsonValueKind.True => "True",
+                JsonValueKind.False => "False",
+                _ => prop.Value.ToString()
+            };
+        }
         return FromFlatDict(dict);
     }
 
diff --git a/tests/ScadaLink.Commons.Tests/Types/DataConnections/OpcUaEndpointConfigSerializerTests.cs b/tests/ScadaLink.Commons.Tests/Types/DataConnections/OpcUaEndpointConfigSerializerTests.cs
index ea3b660..538cbfa 100644
--- a/tests/ScadaLink.Commons.Tests/Types/DataConnections/OpcUaEndpointConfigSerializerTests.cs
+++ b/tests/ScadaLink.Commons.Tests/Types/DataConnections/OpcUaEndpointConfigSerializerTests.cs
@@ -114,6 +114,27 @@ public class OpcUaEndpointConfigSerializerTests
         Assert.Equal("opc.tcp://x:4840", config.EndpointUrl);
     }
 
+    [Fact]
+    public void Deserialize_LegacyWithMixedTypeValues_PreservesAllFields()
+    {
+        // Real-world legacy producer emitted mixed JSON value types:
+        // string for endpoint, number for publishInterval, etc.
+        var legacyJson = """
+            {"endpoint":"opc.tcp://scadalink-opcua:50000","securityMode":"None","publishInterval":1000,"AutoAcceptUntrustedCerts":false,"SessionTimeoutMs":45000}
+            """;
+
+        var (config, isLegacy) = OpcUaEndpointConfigSerializer.Deserialize(legacyJson);
+
+        Assert.True(isLegacy);
+        Assert.Equal("opc.tcp://scadalink-opcua:50000", config.EndpointUrl);
+        Assert.Equal(OpcUaSecurityMode.None, config.SecurityMode);
+        Assert.False(config.AutoAcceptUntrustedCerts);
+        Assert.Equal(45000, config.SessionTimeoutMs);
+        // publishInterval is not a recognized key (the real key is PublishingIntervalMs);
+        // FromFlatDict ignores unknown keys, so PublishingIntervalMs stays at its POCO default of 1000.
+        Assert.Equal(1000, config.PublishingIntervalMs);
+    }
+
     [Theory]
     [InlineData("not json at all")]
     [InlineData("[1,2,3]")]