Dynamics365 PluginRegistration tool throws exception "System.IO.FileLoadException: Could not load file or assembly 'file:///C:\Users\...\xyz.dll' or one of its dependencies."
If you are facing below highlighted error detail with PluginRegistration meaning you are trying to register signed/unsigned dll download over the network.
Quick fix would be to unblock the dll as shown in the below screenshot:
Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'file:///C:\Users\\...\xyz.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515) at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) at System.Reflection.RuntimeAssembly.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark) at System.Reflection.Assembly.LoadFrom(String assemblyFile) at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.LoadAssembly(String path) at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path) at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path) at Microsoft.Crm.Tools.Libraries.RegistrationHelper.RetrievePluginsFromAssembly(String pathToAssembly, String appPath) at Microsoft.Crm.Tools.AssemblyRegistration.PluginRegistrationViewModel.BtnLoadAssemblyClicked()Inner Exception: System.NotSupportedException: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See http://go.microsoft.com/fwlink/?LinkId=155569 for more information.
Quick fix would be to unblock the dll as shown in the below screenshot: