One of our pre-existing custom modules is breaking after I upgraded it to .NET 4.5.1 and loaded it into DNN 8 Engage.

It's a settings control.  Upon clicking the update button on the settings tab the following line runs without error:

objModules.UpdateTabModuleSetting(TabModuleId, "Mode", ddl_Mode.SelectedValue);

Then the DNN framework tries to redirect to the default control of this module (standard behavior).  This is where things break.

A potentially dangerous Request.Path value was detected from the client (:).

Via my browser dev tools I can see that the following malformed URL is being requested:

http://portal/learning-support/submit-support-case,%22http%3A//portal/%22

Is there a setting in webconfig or the DNN framework that is causing this behavior?  This same code works just fine in an older DNN 6 install.

Any guidance appreciated.