When a plugin is triggered, a TypeLoadException error occurrs and it mentions that the plugin cannot load anything from other assemblies in GAC.
OR if you click to open more details about the error you find:
<Message>Could not load type ‘<classname>‘ from assembly ‘<namespace>, Version=126.96.36.199, Culture=neutral, PublicKeyToken=50c75133acc12e13′.</Message>
<Message>Unexpected exception from plug-in (Execute): Plugin.Task.PostTaskUpdate: System.MethodAccessException: Attempt by security transparent method ‘Plugin.Task.PostTaskUpdate.ExecuteMethod(LocalPluginContext)’ to access security critical method ‘Plugin.Common.MyClass..ctor()’ failed.
Assembly ‘Plugin.MyClass, Version=188.8.131.52, Culture=neutral, PublicKeyToken=00b34ecdd47d454b’ is partially trusted, which causes the CLR to make it entirely security transparent regardless of any transparency annotations in the assembly itself. In order to access security critical code, this assembly must be fully trusted.</Message>
Make sure plugin is not registered in sandbox mode.