The error popup is:

Run-time error '424': Object Required

The show  name is typed exactly as it appears in Project Explorer.

Very often when initialising or opening a userform, the debugger will show
the line that does the opening... the .Show line rather than where the
actual problem is. When it does you can continue to "step" through the code
to get to the actual line that is the the bugger in de code.

A word of warning though... when you try to execute the line it will take
you back to the start again! A bit like sliding down that snake but you
don't see the snake coming! It's a bit laborious but as you step through
line by line, set the code marker thingy to the next line you are going to
execute. That's the icon in the debugger that looks like a little blue
arrow. then you can go back to the offending place with a single click. As I
say.. laborious but it does stop the need to go through the code over and
over till you remember where the line is! Another way is ot print out the
code and mark it off line by line.

If I am not wrong, you would have tried to modify the controls structure.
(i.e. you would have added or deleted any control on user form). It may be first
time with you but quite often you experience this problem if once your form is
working perfectly and afterwards you add or modify controls.

To come away with this problem export your module to some place, save and
close VBA project, again open your project, import module and run it. I hope it
would work  perfectly.

In addition to what Lisa pointed out, you may want to check your
References to make sure nothing has gone "Missing". Inside the
VBEditor, from the Tools menu, select References. Make sure nothing is
listed as Missing. If it is, make note of what it is, uncheck it, and
go look for the proper version of the reference. This can happen if
someone ran your code in a higher version of Office than what you have.

A macro attached to one of the buttons on the UserForm
called things that no longer existed. Hence the stop at the macro that called
the form that called the macro. The amazing thing is that I found  it at all!