biblatex style=apa, letter after references of the same year

Question

I'm searching for a solution of my problem for days:

I use biblax with style apa. There is an option for the case if there are more than one references of ONE author in the same year. Than a letter is inserted after the year. My problem is that there is a letter inserted not only if the references are of the same author but only in the same year regardless of the authors.

My document:

\documentclass[11pt,a4paper,twoside]{report}

\usepackage[ngerman]{babel}                                 %

%\usepackage{fancyhdr}
%\usepackage{hyperref} 
\usepackage{csquotes} 
\usepackage[ style=apa,
            url=false,
            doi=false,
            isbn=false,
            backend=biber,
            hyperref=false,
            eprint=false,
            babel=other
]{biblatex}

\addbibresource{C:/Users/Konsi/Documents/Studium/test.bib}  %ort der bib    %

%\bibliography{C:/Users/Konsi/Documents/Studium/test.bib}   %ort der bib    %
\DeclareLanguageMapping{german}{german-apa}
\DefineBibliographyStrings{german}{andothers={et\ al\adddot}} 



\begin{document}

\parencite{Billard2001}\\
\parencite{Wolpert2001487}

\printbibliography
\end{document}

Than the first problem I have: if I compile this the first time with latex->biber->latex->latex than the .bbl file is not built. Therefor I changed the style to authoryear, latex->biber->latex->latex, .bbl file is built. I changed the style back to apa, everthing is apa conform, except the letters after the years.

My .bib is:

% This file was created with JabRef 2.7.2.
% Encoding: Cp1252


@ARTICLE{Billard2001,
  author = {Billard, Aude},
  title = {Learning motir skills by imitation: A biologically inspired robotiv
    model},
  journal = {Cybernetics and Systems},
  year = {2001},
  volume = {32},
  pages = {155-193},
  number = {1-2},
  doi = {10.1080/019697201300001849},
  eprint = {http://www.tandfonline.com/doi/pdf/10.1080/019697201300001849},
  timestamp = {2012.07.08},
  url = {http://www.tandfonline.com/doi/abs/10.1080/019697201300001849}
}


@ARTICLE{Wolpert2001487,
  author = {Daniel M Wolpert and Zoubin Ghahramani and J.Randall Flanagan},
  title = {Perspectives and problems in motor learning},
  journal = {Trends in Cognitive Sciences},
  year = {2001},
  volume = {5},
  pages = {487 - 494},
  number = {11},
  doi = {10.1016/S1364-6613(00)01773-3},
  issn = {1364-6613},
  keywords = {movement},
  timestamp = {2012.07.07},
  url = {http://www.sciencedirect.com/science/article/pii/S1364661300017733}
}

Could anyone please help me? The second problem is caused by the first maybe, but if so, how can I compile with style=apa?

Thanks a lot!

Edit: Used versions are (I'm not really sure, what is useful and what not): The apa.dbx is missing. is this maybe a problem? I installed the biblatex-apa package with the MiKTeX 2.9 Package Manager.

*File List*
report.cls    2007/10/19 v1.4h Standard LaTeX document class   

size11.clo    2007/10/19 v1.4h Standard LaTeX file (size option) 

babel.sty    2008/07/08 v3.8m The Babel package  

bblopts.cfg    2006/07/31 v1.0 MiKTeX 'babel' configuration 

ngermanb.ldf    2008/07/06 v2.6n new German support from the babel system 

csquotes.sty    2011/10/22 v5.1d context-sensitive quotations 

etoolbox.sty    2011/01/03 v2.1 e-TeX tools for LaTeX

etex.sty    1998/03/26 v2.0 eTeX basic definition package (PEB)   

keyval.sty    1999/03/16 v1.13 key=value parser (DPC) 

csquotes.def    2011/10/22 v5.1d csquotes generic definitions 

csquotes.cfg     

biblatex.sty    2012/07/01 v2.0 programmable bibliographies (PK/JW/AB) 

biblatex2.sty    2012/07/01 v2.0 programmable bibliographies (biber) (PK/JW/AB)


kvoptions.sty    2010/12/23 v3.10 Keyval support for LaTeX options (HO)  

ltxcmds.sty    2011/04/18 v1.20 LaTeX kernel commands for general use (HO) 

kvsetkeys.sty    2011/04/07 v1.13 Key value parser (HO) 

infwarerr.sty    2010/04/08 v1.3 Providing info/warning/message (HO) 

etexcmds.sty    2011/02/16 v1.5 Prefix for e-TeX command names (HO) 

ifluatex.sty    2010/03/01 v1.3 Provides the ifluatex switch (HO) 

logreq.sty    2010/08/04 v1.0 xml request logger   

logreq.def    2010/08/04 v1.0 logreq spec v1.0   

ifthen.sty    2001/05/26 v1.1c Standard LaTeX ifthen package (DPC)

url.sty    2006/04/12  ver 3.3  Verb mode for urls, etc.   

blx-dm.def

apa.dbx 

biblatex-dm.cfg 

blx-compat.def    2012/07/01 v2.0 biblatex compatibility (PK/JW/AB) 

biblatex.def     

standard.bbx    2012/07/01 v2.0 biblatex bibliography style (PK/JW/AB)

apa.bbx    2012/07/01 v5.0 APA biblatex references style

apa.cbx    2012/07/01 5.0 APA biblatex citation style 

biblatex.cfg       

german.lbx    2012/07/01 v2.0 biblatex localization (PK/JW/AB) 

german-apa.lbx    2012/07/01 v5.0 APA biblatex localisation 

ngerman.lbx    2012/07/01 v2.0 biblatex localization (PK/JW/AB) 
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