I tried changing the macro settings and then restarting the program. Am I doing this right? Thanks for the help!
Macro security shouldn't need to be lowered (as long as macros are enabled and VBA code is allowed to run) - looks like we're going to have to ship a hotfix release this week...
The stacktrace in the log implies we're incorrectly handling a COM object in that part of the codebase. Thanks for the report :)
2018-11-25 21:37:01.8147;WARN-2.3.0.4221;Rubberduck.UI.UnitTesting.Commands.AddTestModuleCommand;Unable to add test module. An exception was thrown.;
2018-11-25 21:37:01.8277;WARN-2.3.0.4221;Rubberduck.UI.UnitTesting.Commands.AddTestModuleCommand;System.Runtime.InteropServices.InvalidComObjectException: COM object that has been separated from its underlying RCW cannot be used.
at System.StubHelpers.StubHelpers.GetCOMIPFromRCW(Object objSrc, IntPtr pCPCMD, IntPtr& ppTarget, Boolean& pfNeedsRelease)
at Microsoft.Vbe.Interop._VBProject.get_Protection()
at Rubberduck.VBEditor.SafeComWrappers.VBA.VBProject.get_Protection() in C:\projects\rubberduck\Rubberduck.VBEditor.VBA\SafeComWrappers\VB\VBProject.cs:line 49
at Rubberduck.VBEditor.SafeComWrappers.VBA.VBProject.get_VBComponents() in C:\projects\rubberduck\Rubberduck.VBEditor.VBA\SafeComWrappers\VB\VBProject.cs:line 47
at Rubberduck.UI.UnitTesting.Commands.AddTestModuleCommand.ExecuteInternal(IVBProject project, Declaration projectDeclaration) in C:\projects\rubberduck\Rubberduck.Core\UI\UnitTesting\Commands\AddTestModuleCommand.cs:line 195;System.Runtime.InteropServices.InvalidComObjectException: COM object that has been separated from its underlying RCW cannot be used.
at System.StubHelpers.StubHelpers.GetCOMIPFromRCW(Object objSrc, IntPtr pCPCMD, IntPtr& ppTarget, Boolean& pfNeedsRelease)
at Microsoft.Vbe.Interop._VBProject.get_Protection()
at Rubberduck.VBEditor.SafeComWrappers.VBA.VBProject.get_Protection() in C:\projects\rubberduck\Rubberduck.VBEditor.VBA\SafeComWrappers\VB\VBProject.cs:line 49
at Rubberduck.VBEditor.SafeComWrappers.VBA.VBProject.get_VBComponents() in C:\projects\rubberduck\Rubberduck.VBEditor.VBA\SafeComWrappers\VB\VBProject.cs:line 47
at Rubberduck.UI.UnitTesting.Commands.AddTestModuleCommand.ExecuteInternal(IVBProject project, Declaration projectDeclaration) in C:\projects\rubberduck\Rubberduck.Core\UI\UnitTesting\Commands\AddTestModuleCommand.cs:line 195
As a workaround you could add the module without involving Rubberduck. The module will still be discovered after parsing then.

Adding test module manually does work though, and tests run fine.
Thanks, you folks are great. I hope to get on your level one day.