Oh, I see that this is caused by __namespace__ = "System.Windows.Forms". Can you explain how GetPreferredSize gets called? The traceback is likely broken due to callback to CLR?

Can you explain how GetPreferredSize gets called?

GetPreferredSize() here is an overload for virtual method of System.Windows.Forms.GroupBox class. It is called from .NET code inside self.Controls.Add() method of System.Windows.Forms.Form class. So callstack is: Python -> .NET ->Python GetPreferredSize().

It is really difficult to debug Python script with such traceback. Situation is slightly better if Exception was thrown - by changing in source "WinForms.GroupBox.GetPreferredSize" to "WinForms.GroupBox" I get:

Traceback (most recent call last):
File "t1.py", line 32, in
main()
File "t1.py", line 24, in main
form = StatusWindow()
File "t1.py", line 18, in init
self.Controls.Add(_StagesGroup())
System.InvalidCastException: cannot convert object to target type
at Python.Runtime.PyObject.AsManagedObject (System.Type t) [0x00016] in <90f56f11bd4c4fbf9e32746d89de693d>:0
at Python.Runtime.PythonDerivedType.InvokeMethod[T] (Python.Runtime.IPythonDerivedType obj, System.String methodName, System.String origMethodName, System.Object[] args) [0x00107] in <90f56f11bd4c4fbf9e32746d89de693d>:0
at System.Windows.Forms._StagesGroup.GetPreferredSize (System.Drawing.Size ) [0x00021] in <9b9523702e53439da45ef812ca9b9c8a>:0

Here I see at least method name "System.Windows.Forms._StagesGroup.GetPreferredSize" but also no line number.

Valid line numbers for the C# code in a stacktrace could appears only if .dll files are paired with .pdb files.
Please firstly verify that your setup contains .pdb files at the right location.

@dmitriyse
The problem is that I can't generate .pdb for python code.

@denfromufa
My python script implements event handler or virtual method. At some points .NET class calls it through Pythonnet. I think you use in pythonnet something like PyObject_CallObject() for calling python callback, but I can't find a exact place where it is done.

Now, imagine python callback function got some interpreter error or exception. In this case PyObject_CallObject() returns NULL. Then pythonnet creates new exception object, isn't it ?
At this point pythonnet can fetch a traceback from exception and reattach it to new exception.

But at the moment pythonnet doesn't save traceback, so not possible to localize the source of error. I think this can be improved.
Do you know where in pythonnet source python virtual method or event callback are called ?

The Python method gets called via PyObject.Invoke in pyobject.cs (called from PythonDerivedType.InvokeMethod in classderived.cs). You could fetch the Python traceback in there and use that to add more information to the .net PythonException thrown.

I think this is fixed now.