What is the meaning of an error message "can't find file ***.vrb"? (A fix now known, a full explanation still missing)

My system:

• Windows 10
• WinEdt 10.2.
• MikTeX 2.9.

The problem:

I am editing a set of powerdot slides named Luku16.tex. All of sudden I started getting these strange error messages when trying to compile within WinEdt (TeX-> DVI -> LaTeX). T

 LaTeX Warning: Reference `lastslide' on page 1 undefined on input line 55.

! I can't find file `Luku16.vrb'.
<argument> ...pd@body else input jobname .vrb 
                                                  fi end {pd@minipage}
l.55 end{slide}
                
Please type another input file name: 

I am clueless as about the meaning of this. This is a previously uncompiled source file, so LaTeX has not created any auxiliaries yet. What am I supposed to type in here?

I think I managed to create this same behavior once by accidentally creating a new .tex-file in a folder that contained residues of another with the same name. This is not the cause here. I renamed the file to exclude this possibility.

Symptoms:

• When I look at the listing of this folder, there is a curious file
  with the name Luku16.synctex(busy) of filesize 0 bytes.
• The other slide sets in this same folderwork just fine (they do have all the auxiliary files in place already though).

---

Can anyone diagnose this? What should I do to fix this?

---

More testing:

The early parts (is it called the preamble or what?)

documentclass{powerdot}
usepackage{lmodern}
usepackage[utf8]{inputenc}
%usepackage[latin1]{inputenc}
usepackage[T1]{fontenc}
usepackage[finnish]{babel}
usepackage{psfrag}
usepackage{xcolor}
usepackage{amsmath,amssymb}
usepackage{hyperref}
usepackage{animate}


begin{document}

I am close to a solution in that I have identified a line in the source file causing this. The error message was generated at the end of the title slide. Here’s a comparison. When the title slide has the source

begin{slide}{Analyysi II B}
Potenssisarjojen manipulointia ja sovelluksia, Luku 16
end{slide}

The problem does NOT appear. I would prefer to use quotes (because this is not really chapter 16, it’s rather "chapter 16" as the book only has 15 chapters. So initially my source looked like

begin{slide}{Analyysi II B}
Potenssisarjojen manipulointia ja sovelluksia, "Luku 16"
end{slide}

And that seems to somehow be too much for LaTeX to handle. Bizarre?

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Ok, so it is the quotes. I have grown used to using "normal" quotes in place of the “US style” quotes. It is bread and butter to fix the resulting spacing errors with /. As the next test I tried the source

begin{slide}{Analyysi II B}
Potenssisarjojen manipulointia ja sovelluksia, "Luku 16"/
end{slide}

And this also works! No errors. Of course, it is a bit strange to have to add extra whitespace to a place where none is needed, but…

The main problem is solved.

I am still curious as to exactly why this happens, so leaving this open in case somebody feels like writing an explanation as an answer 🙂