Cookies are creating the width problem until they are refreshed.
This MOD appears to be your problem because it has nowrap and the line is too long. To make it worse it's also in two sections / strings with nowrap that follow each other.
Also this code is from "Veiw Source", Your script will have VARS in it.
<table width="100%" cellspacing="0">
<tr>
<td class="nav" nowrap="nowrap">
<img src="./styles/black_pearl/imageset/post_page_find.png" alt="Thema beobachten" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&watch=topic&start=0" title="Thema beobachten">Thema beobachten</a> | <img src="./styles/black_pearl/imageset/post_book.png" alt="Lesezeichen setzen" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&start=0&st=0&sk=t&sd=a&bookmark=1" title="Lesezeichen setzen">Lesezeichen setzen</a> | <img src="./styles/black_pearl/imageset/post_print.png" alt="Druckansicht" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&start=0&st=0&sk=t&sd=a&view=print" title="Druckansicht">Druckansicht</a> | <img src="./styles/black_pearl/imageset/post_email.png" alt="Thema weiterempfehlen" style="vertical-align:middle;" /> <a href="./memberlist.php?mode=email&t=3432" title="Thema weiterempfehlen">Thema weiterempfehlen</a> </td>
<td class="nav" align="right" nowrap="nowrap"><img src="./styles/black_pearl/imageset/post_previous.png" alt="Vorheriges Thema" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&view=previous">Vorheriges Thema</a> | <img src="./styles/black_pearl/imageset/post_new.png" alt="Erster ungelesener Beitrag" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&view=unread#unread">Erster ungelesener Beitrag</a> | <a href="./viewtopic.php?f=200&t=3432&view=next">Nächstes Thema</a> <img src="./styles/black_pearl/imageset/post_next.png" alt="Nächstes Thema" style="vertical-align:middle;" /></td>
</tr>
</table>
In the script (header / wherever) find and remove:
</td>
<td class="nav" align="right" nowrap="nowrap">
That should fix one problem, and you may just want to try that.
You could also add a marquee (As in the example below) to it so that it scrolls - and that should fix the width problem if it gets too long just with normal posting.
<table width="100%" cellspacing="0">
<tr>
<td class="nav" >
<marquee width="900px">
<img src="./styles/black_pearl/imageset/post_pnowrap="nowrap"age_find.png" alt="Thema beobachten" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&watch=topic&start=0" title="Thema beobachten">Thema beobachten</a> | <img src="./styles/black_pearl/imageset/post_book.png" alt="Lesezeichen setzen" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&start=0&st=0&sk=t&sd=a&bookmark=1" title="Lesezeichen setzen">Lesezeichen setzen</a> | <img src="./styles/black_pearl/imageset/post_print.png" alt="Druckansicht" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&start=0&st=0&sk=t&sd=a&view=print" title="Druckansicht">Druckansicht</a> | <img src="./styles/black_pearl/imageset/post_email.png" alt="Thema weiterempfehlen" style="vertical-align:middle;" /> <a href="./memberlist.php?mode=email&t=3432" title="Thema weiterempfehlen">Thema weiterempfehlen</a>
<img src="./styles/black_pearl/imageset/post_previous.png" alt="Vorheriges Thema" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&view=previous">Vorheriges Thema</a> | <img src="./styles/black_pearl/imageset/post_new.png" alt="Erster ungelesener Beitrag" style="vertical-align:middle;" /> <a href="./viewtopic.php?f=200&t=3432&view=unread#unread">Erster ungelesener Beitrag</a> | <a href="./viewtopic.php?f=200&t=3432&view=next">Nächstes Thema</a> <img src="./styles/black_pearl/imageset/post_next.png" alt="Nächstes Thema" style="vertical-align:middle;" />
</marquee>
</td>
</tr>
</table>
If you opt for the marquee, then note this change also: <td class="nav" >