The first byte of the proxy settings binary was '60' but should have been 0x46.

Problem description

Received Error:
The first byte of the proxy settings binary was '60' but should have been 0x46.

But DefaultConnectionSettings and SavedLegacySettings is a completely fresh entry

Verbose logs

AUSFÜHRLICH: [xxx]:                            [[ProxySettings]ManualProxy] Test-TargetResource: Checking the CurrentUser proxy settings to ensure 'Present'.
AUSFÜHRLICH: [xxx]:                            [[ProxySettings]ManualProxy] Test-TargetResource: Checking that the CurrentUser proxy settings 'DefaultConnectionSettings' are in the desired state.
AUSFÜHRLICH: [xxx]: LCM:  [ BeendenTesten   ]  [[ProxySettings]ManualProxy]  in 0.1750 Sekunden.
Die PowerShell DSC-Ressource "DSC_ProxySettings" konnte die Funktion "Test-TargetResource" nicht ausführen. Fehlermeldung: "System.InvalidOperationException: The first byte of the proxy settings binary was '60' but should have been 0x46." 
    + CategoryInfo          : InvalidOperation: (:) [], CimException
    + FullyQualifiedErrorId : ProviderOperationExecutionFailure
    + PSComputerName        : xxx

DSC configuration

ProxySettings ManualProxy
        {
            Target = 'CurrentUser'
            Ensure = 'Present'
            EnableAutoDetection = $false
            EnableAutoConfiguration = $false
            EnableManualProxy = $true
            ProxyServer = 'proxy.xxx.local:3128'
            ProxyServerExceptions = '*.xxx.local', '*.e-xxx.de', '*.xx.xxxx.de'
            ProxyServerBypassLocal = $true
        }

Suggested solution

Allow updating the proxy settings

Operating system the target node is running

OsName               : Microsoft Windows 10 Pro
OsOperatingSystemSKU : 48
OsArchitecture       : 64-Bit
WindowsVersion       : 2009
WindowsBuildLabEx    : 19041.1.amd64fre.vb_release.191206-1406
OsLanguage           : de-DE
OsMuiLanguages       : {de-DE}

PowerShell version and build the target node is running

Name                           Value
----                           -----
PSVersion                      5.1.19041.3031
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.19041.3031
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

NetworkingDsc version

NetworkingDsc 9.0.0   C:\Program Files\WindowsPowerShell\Modules\NetworkingDsc\9.0.0\NetworkingDsc.psd1

Start with the ProxySettings ManualProxy resource and its Test-TargetResource entry point, focusing on the CurrentUser DefaultConnectionSettings and SavedLegacySettings values. Reproduce the failure on the reported Windows 10 and PowerShell 5.1 environment, then verify that the resource can update the proxy settings without the first-byte error.