I am having the same problem in Joomla 3. It appears to mostly affect Firefox.
You mentioned URL-rewriting is probably the culprit. Does anything look incorrect to you? It still seems to submit half the time, but this error appears intermittently.
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Joomla! Rule 1" stopProcessing="true">
<match url="^(.*)$" ignoreCase="false" />
<conditions logicalGrouping="MatchAny">
<add input="{QUERY_STRING}" pattern="base64_encode[^(]*\([^)]*\)" ignoreCase="false" />
<add input="{QUERY_STRING}" pattern="(>|%3C)([^s]*s)+cript.*(<|%3E)" />
<add input="{QUERY_STRING}" pattern="GLOBALS(
\[|\%[0-9A-Z]{0,2})" ignoreCase="false" />
<add input="{QUERY_STRING}" pattern="_REQUEST(
\[|\%[0-9A-Z]{0,2})" ignoreCase="false" />
</conditions>
<action type="CustomResponse" url="index.php" statusCode="403" statusReason="Forbidden" statusDescription="Forbidden" />
</rule>
<rule name="Joomla! Rule 2">
<match url="(.*)" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{URL}" pattern="^/index.php" ignoreCase="true" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="false" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="false" negate="true" />
</conditions>
<action type="Rewrite" url="index.php" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>