diff --git a/BlazorApp/Components/Pages/Developers.razor b/BlazorApp/Components/Pages/Developers.razor
index 9f69b71..abfaf63 100644
--- a/BlazorApp/Components/Pages/Developers.razor
+++ b/BlazorApp/Components/Pages/Developers.razor
@@ -107,6 +107,10 @@
                             {
                                 <span class="username-status username-taken">&#10007; Username is already taken</span>
                             }
+                            else if (_usernameCheckState == UsernameCheckState.Error)
+                            {
+                                <span class="username-status username-error">&#9888; Could not check availability — you can still submit</span>
+                            }
                             <ValidationMessage For="() => _application.DesiredUsername" />
                         </div>
 
@@ -209,7 +213,7 @@
                     {
                         <div class="dev-error">@_errorMessage</div>
                     }
-                    <button type="submit" class="dev-btn dev-btn-primary" disabled="@_submitting">
+                    <button type="submit" class="dev-btn dev-btn-primary" disabled="@(_submitting || _usernameCheckState == UsernameCheckState.Taken)">
                         @if (_submitting)
                         {
                             <span class="btn-spinner"></span>
@@ -240,7 +244,7 @@
 
     private readonly string[] _availablePlatforms = { "Windows", "macOS", "Linux", "Android", "iOS", "Other" };
 
-    private enum UsernameCheckState { None, Checking, Available, Taken }
+    private enum UsernameCheckState { None, Checking, Available, Taken, Error }
 
     private void SelectRole(ApplicationRole role)
     {
@@ -279,7 +283,12 @@
 
             if (!token.IsCancellationRequested)
             {
-                _usernameCheckState = available ? UsernameCheckState.Available : UsernameCheckState.Taken;
+                _usernameCheckState = available switch
+                {
+                    true => UsernameCheckState.Available,
+                    false => UsernameCheckState.Taken,
+                    null => UsernameCheckState.Error
+                };
                 StateHasChanged();
             }
         }
diff --git a/BlazorApp/Endpoints/DeveloperEndpoints.cs b/BlazorApp/Endpoints/DeveloperEndpoints.cs
index 46c6fdc..7893da1 100644
--- a/BlazorApp/Endpoints/DeveloperEndpoints.cs
+++ b/BlazorApp/Endpoints/DeveloperEndpoints.cs
@@ -12,7 +12,9 @@ public static class DeveloperEndpoints
         group.MapGet("/check-username/{username}", async (string username, DeveloperApplicationService service) =>
         {
             var available = await service.CheckUsernameAsync(username);
-            return Results.Ok(new { available });
+            if (available is null)
+                return Results.Problem("Unable to verify username availability", statusCode: 503);
+            return Results.Ok(new { available = available.Value });
         });
 
         group.MapPost("/apply", async (DeveloperApplication application, DeveloperApplicationService service) =>
diff --git a/BlazorApp/Services/DeveloperApplicationService.cs b/BlazorApp/Services/DeveloperApplicationService.cs
index 1b1c48b..12cf8df 100644
--- a/BlazorApp/Services/DeveloperApplicationService.cs
+++ b/BlazorApp/Services/DeveloperApplicationService.cs
@@ -17,21 +17,30 @@ public class DeveloperApplicationService
         _logger = logger;
     }
 
-    public async Task<bool> CheckUsernameAsync(string username)
+    /// <summary>
+    /// Checks username availability. Returns: true = available, false = taken, null = error/unknown.
+    /// </summary>
+    public async Task<bool?> CheckUsernameAsync(string username)
     {
         try
         {
             var response = await _httpClient.GetAsync($"/api/auth/check-username/{Uri.EscapeDataString(username)}");
             if (!response.IsSuccessStatusCode)
-                return false;
+            {
+                _logger.LogWarning("Username check returned {StatusCode} for {Username}", response.StatusCode, username);
+                return null;
+            }
 
             var result = await response.Content.ReadFromJsonAsync<JsonElement>();
-            return result.TryGetProperty("available", out var available) && available.GetBoolean();
+            if (result.TryGetProperty("available", out var available))
+                return available.GetBoolean();
+
+            return null;
         }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Error checking username availability for {Username}", username);
-            return false;
+            return null;
         }
     }
 
diff --git a/BlazorApp/wwwroot/developers-styles.css b/BlazorApp/wwwroot/developers-styles.css
index bc4590b..6519f7c 100644
--- a/BlazorApp/wwwroot/developers-styles.css
+++ b/BlazorApp/wwwroot/developers-styles.css
@@ -186,6 +186,29 @@
     margin-top: 0.3rem;
 }
 
+/* Username status */
+.username-status {
+    display: block;
+    font-size: 0.82rem;
+    margin-top: 0.35rem;
+}
+
+.username-checking {
+    color: rgba(255, 255, 255, 0.5);
+}
+
+.username-available {
+    color: #34d399;
+}
+
+.username-taken {
+    color: #f87171;
+}
+
+.username-error {
+    color: #fbbf24;
+}
+
 /* Validation messages */
 .validation-message {
     color: #f87171;