Hi Adamsmith,
thank you very much for your quick reply! Really appreciate your precious help. Yes, I tried with an empty document and I have same mess.

Many thanks again.

In the third citation, Zotero adds a second author before the et al - Morley, King, et al. - as if using disambiguate-add-names - but without any apparent reason.

Using the processor embedded in Zotero 2.1b7, I am unable to reproduce this failure. I'll try installing the latest beta to see if it can be reproduced in Zotero.

Hi fbennett, thank you very much for helping with that. Cheers.

Hi fbennett,
will a downgrade to some 2.1 beta make me continue working on my stuff? What do you advice?
Many thanks.

Hi Adamsmith
thanks for the advices. Any link you can post where I can learn more about disabling disambiguation functionality in a style?

Cheers,
nik

(for what it's worth, remove the disambiguate-add-names option from the citation section of the style http://www.zotero.org/support/csl_simple_edits but as this is fixed in the next version of Zotero I wouldn't do that unless you're in a big hurry).

I did a temp style, I removed from it all the disambiguate-names things but nothing changes. I still get the same problem. Any thoughts? Below the temp style I created.

<?xml version="1.0" encoding="UTF-8"?>
<style xmlns="http://purl.org/net/xbiblio/csl" class="in-text" xml:lang="en">
<info>
<title>TempStyleNik</title>
<id>http://www.zotero.org/styles/nik</id>;
<link href="http://www.zotero.org/styles/nik"/>
<author>
<name>Rintze Zelle</name>
<uri>http://forums.zotero.org/account/831/</uri>;
</author>
<contributor>
<name>Sebastian Karcher</name>
</contributor>
<category term="geology"/>
<link href="http://www.geosociety.org/pubs/geoguid5.htm" rel="documentation"/>
<updated>2010-07-26T02:21:30+00:00</updated>
<rights>This work is licensed under a Creative Commons Attribution-Share Alike 3.0 License: http://creativecommons.org/licenses/by-sa/3.0/</rights>;
</info>
<macro name="editor-translator">
<names variable="editor translator" prefix="(" suffix=")" delimiter=", ">
<name and="symbol" initialize-with=". " delimiter=", "/>
<label form="short" prefix=", " text-case="capitalize-first" suffix="."/>
</names>
</macro>
<macro name="author">
<names variable="author">
<name name-as-sort-order="all" and="text" sort-separator=", " initialize-with="."
delimiter=", " delimiter-precedes-last="always"/>
<label form="short" prefix=" (" suffix=".)" text-case="capitalize-first"/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text variable="title"/>
</substitute>
</names>
</macro>
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", " initialize-with="."/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<choose>
<if type="book">
<text variable="title" form="short" font-style="italic"/>
</if>
<else>
<text variable="title" form="short" quotes="true"/>
</else>
</choose>
</substitute>
</names>
</macro>
<macro name="publisher">
<group delimiter=", ">
<text variable="publisher"/>
<text variable="publisher-place"/>
</group>
</macro>
<citation>
<option name="et-al-min" value="3"/>
<option name="et-al-use-first" value="1"/>
<option name="et-al-subsequent-min" value="3"/>
<option name="et-al-subsequent-use-first" value="1"/>
<option name="disambiguate-add-year-suffix" value="true"/>
<layout prefix="(" suffix=")" delimiter="; ">
<group delimiter=", ">
<text macro="author-short"/>
<date variable="issued" >
<date-part name="year"/>
</date>
<group>
<label variable="locator" suffix="." form="short"/>
<text variable="locator" prefix=" "/>
</group>
</group>
</layout>
</citation>
<bibliography>
<option name="hanging-indent" value="true"/>
<option name="et-al-min" value="15"/>
<option name="et-al-use-first" value="15"/>
<sort>
<key macro="author-short"/>
<key variable="title"/>
</sort>
<layout suffix=".">
<group delimiter=", ">
<text macro="author"/>
<date variable="issued" suffix=",">
<date-part name="year"/>
</date>
</group>
<choose>
<if type="report">
<text variable="title" prefix=" " suffix=":"/>
<text variable="publisher" prefix=" " suffix=","/>
<text variable="collection-title" prefix=" "/>
</if>
<else-if type="book">
<group suffix=":">
<text variable="title" prefix=" "/>
<text macro="editor-translator" prefix=" "/>
</group>
<text prefix=" " suffix="." macro="publisher"/>
</else-if>
<else-if type="chapter">
<text variable="title" prefix=" "/>
<group class="container" prefix=", ">
<text term="in" text-case="lowercase" font-style="italic"/>
<names variable="editor translator" prefix=" " suffix="," delimiter=", " >
<name and="text" sort-separator=", " initialize-with="." name-as-sort-order="all"/>
<label form="short" prefix=" " suffix="." text-case="lowercase"/>
</names>
<text variable="container-title" prefix=" " suffix=","/>
<text variable="collection-title" prefix=" " suffix=","/>
<group suffix=".">
<text macro="publisher" prefix=" "/>
<group prefix=", ">
<text variable="page" prefix="p. "/>
</group>
</group>
</group>
</else-if>
<else-if type="map">
<text variable="title" prefix=" "/>
<text variable="collection-title" prefix=": "/>
<text variable="note" prefix=", scale "/>
</else-if>
<else-if type="thesis">
<text variable="title" prefix=" "/>
<text variable="genre" prefix=" [" suffix="]"/>
<group prefix=": " delimiter=", ">
<!--text variable="place" -->
<text variable="publisher"/>
<!--text variable="number-of-pages" suffix=" p."-->
</group>
</else-if>
<else>
<group suffix=":">
<text variable="title" prefix=" "/>
<text macro="editor-translator" prefix=" "/>
</group>
<group class="container" prefix=" " suffix=".">
<text variable="container-title" suffix=","/>
<group prefix=" ">
<text variable="volume" prefix="v. "/>
<text variable="issue" prefix=", no. "/>
</group>
<text variable="page" prefix=", p. "/>
</group>
</else>
</choose>
</layout>
</bibliography>
</style>

if you want to you can try this:
https://gist.github.com/raw/829476/d2c4a96fa3204428acf58877b86440558a7b8111/nik.csl
but fbennett is right.