can't write on file xx.aux...

there seems to be a problem with the last update with some file names
involving numbers.

the below file is called "X1.1.tex":
----X

\documentclass[a4paper, 12pt, reqno]{article}

\begin{document}

\title{Title}

\author{Author}

\maketitle\thispagestyle{empty}
%%%---------------------------------------------------------------------------------------
\begin{abstract}
    \noindent We analyze\ldots
\end{abstract}
%%%---------------------------------------------------------------------------------------

\end{document}

----X
and generates the following log:

This is pdfTeX, Version 3.14159265-2.6-1.40.16 (MiKTeX 2.9 64-bit)
(preloaded format=latex 2015.12.9)  9 DEC 2015 09:20
entering extended mode
**C:/Users/pauls/Dox/Academic/Teaching/Mikro/lectures/X1.1.tex
(C:/Users/pauls/Dox/Academic/Teaching/Mikro/lectures/X1.1.tex
LaTeX2e <2015/10/01> patch level 2
Babel <3.9m> and hyphenation patterns for 69 languages loaded.
("C:\Program Files\MiKTeX 2.9\tex\latex\base\article.cls"
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
("C:\Program Files\MiKTeX 2.9\tex\latex\base\size12.clo"
File: size12.clo 2014/09/29 v1.4h Standard LaTeX file (size option)
)
\c@part=\count79
\c@section=\count80
\c@subsection=\count81
\c@subsubsection=\count82
\c@paragraph=\count83
\c@subparagraph=\count84
\c@figure=\count85
\c@table=\count86
\abovecaptionskip=\skip41
\belowcaptionskip=\skip42
\bibindent=\dimen102
)

LaTeX Warning: Unused global option(s):
    [reqno].

No file X1.1.aux.
! I can't write on file `X1.1.aux'.
\document ...ate \openout \@mainaux \jobname .aux
                                                  \immediate \write
\@mainau...
l.3 \begin{document}

Please type another output file name:
! Emergency stop.
\document ...ate \openout \@mainaux \jobname .aux
                                                  \immediate \write
\@mainau...
l.3 \begin{document}

End of file on the terminal!


Here is how much of TeX's memory you used:
 200 strings out of 493635
 2199 string characters out of 3143770
 52601 words of memory out of 3000000
 3675 multiletter control sequences out of 15000+200000
 3938 words of font info for 15 fonts, out of 3000000 for 9000
 1025 hyphenation exceptions out of 8191
 22i,0n,21p,163b,36s stack positions out of 5000i,500n,10000p,200000b,50000s
No pages of output.

before the last update, file names of this form did compile fine.  if i
rename the file to "X1.01.tex", everything is OK.

thank you & best

 - paul.

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