urlrewritefilter: imposible to get two redirections

I'm trying to make friendly URL's with urlrewritefilter on a java web page.

At first, I made this:

<!-- redirects url.com/something -->
<rule> 
    <from>^/([a-z]+)$</from>
    <to>/motor/index.jsp?empresa=$1</to>
</rule>

<!-- redirects url.com/something/ -->
<rule>
    <from>^/([a-z]+)/$</from>
    <to>/motor/index.jsp?empresa=$1</to>
</rule>

but, when I try to add some other rule, It does'nt works. Any web page I try to enter will cause this error:

org.tuckey.web.filters.urlrewrite.NormalRewrittenUrl.doRewrite(NormalRewrittenUrl.java:213)
org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:171)
org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:145)
org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:92)
org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:394)

These are the orthers rules I tryed to introduce:

<!-- redirects url.com/(numbers) -->
<rule>
    <form>^/([0-9]+)$</form>
    <to>/motor/login.jsp</to>
</rule>

<!-- redirects url.com/start-sesion -->
<!-- This is the rule I want to get working. The previous rule was just a test -->
<rule>
    <form>^/start-sesion$</form>
    <to>/motor/login.jsp</to>
</rule>

Any way. None of these rules worked.

Some ideas? :P