[BUG] Check for updates fails when a proxy is enabled

[newyorker123]

Please confirm these before moving forward

• I have searched for my issue and have not found a work-in-progress/duplicate/resolved issue.
• I have tested that this issue has not been fixed in the latest (beta or stable) release.
• I have checked the FAQ section for solutions.
• This issue is about a bug (if it is not, please use the correct template).

UniGetUI Version

2026.1.0

Windows version, edition, and architecture

Windows 11 pro 25H2

Describe your issue

When a proxy is configured in the settings, UniGetUI fails to check for updates. The process returns an error message: "This option is not supported for a relative URI."

If the proxy is disabled, the update check works as expected.

Steps to reproduce the issue

1. Open UniGetUI and go to Settings.
2. Navigate to the Proxy section and configure a valid proxy server.
3. Click "Check for updates" button
4. Observe the error message: "This option is not supported for a relative URI".

UniGetUI Log

[2026/3/15 10:22:03] Productinfo updater source failed. Falling back to legacy GitHub updater source.
[2026/3/15 10:22:03] System.InvalidOperationException: This operation is not supported for a relative URI.
                        at System.Uri.get_Scheme()
                        at System.Net.Http.HttpConnectionPoolManager.SendAsync(HttpRequestMessage, Boolean, Boolean, CancellationToken)
                        at System.Net.Http.HttpConnectionHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage, CancellationToken)
                        at System.Net.Http.DiagnosticsHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage, CancellationToken)
                        at System.Net.Http.Metrics.MetricsHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.DecompressionHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpClient.GetStringAsyncCore(HttpRequestMessage, CancellationToken)
                        at UniGetUI.AutoUpdater.CheckForUpdatesFromProductInfo(UpdaterOverrides updaterOverrides) in D:\a\UniGetUI\UniGetUI\src\UniGetUI\AutoUpdater.cs:line 204
                        at UniGetUI.AutoUpdater.GetUpdateCandidate(UpdaterOverrides updaterOverrides) in D:\a\UniGetUI\UniGetUI\src\UniGetUI\AutoUpdater.cs:line 177
[2026/3/15 10:22:03] Using legacy GitHub updater source due to registry override.
[2026/3/15 10:22:03] An error occurred while checking for updates:
[2026/3/15 10:22:03] System.InvalidOperationException: This operation is not supported for a relative URI.
                        at System.Uri.get_Scheme()
                        at System.Net.Http.HttpConnectionPoolManager.SendAsync(HttpRequestMessage, Boolean, Boolean, CancellationToken)
                        at System.Net.Http.HttpConnectionHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage, CancellationToken)
                        at System.Net.Http.DiagnosticsHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage, CancellationToken)
                        at System.Net.Http.Metrics.MetricsHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.DecompressionHandler.SendAsync(HttpRequestMessage, Boolean, CancellationToken)
                        at System.Net.Http.HttpClient.GetStringAsyncCore(HttpRequestMessage, CancellationToken)
                        at UniGetUI.AutoUpdater.CheckForUpdatesFromLegacyGitHub(UpdaterOverrides updaterOverrides) in D:\a\UniGetUI\UniGetUI\src\UniGetUI\AutoUpdater.cs:line 261
                        at UniGetUI.AutoUpdater.GetUpdateCandidate(UpdaterOverrides updaterOverrides) in D:\a\UniGetUI\UniGetUI\src\UniGetUI\AutoUpdater.cs:line 183
                        at UniGetUI.AutoUpdater.CheckAndInstallUpdates(Window window, InfoBar banner, Boolean Verbose, Boolean AutoLaunch, Boolean ManualCheck) in D:\a\UniGetUI\UniGetUI\src\UniGetUI\AutoUpdater.cs:line 101

Package Managers Logs

Not related

Relevant information

I am aware that there have been several discussions regarding the 'check for update' functionality recently. However, this specific issue appears to be uniquely triggered by the proxy configuration, and I haven't found any existing reports mentioning this particular 'relative URI' error.

Screenshots and videos