Groovy & Grails: Error intercepting locale change: Locale part "robots.txt" contains invalid characters

Groovy & Grails: Error intercepting locale change: Locale part
"robots.txt" contains invalid characters

I got a grails application running on http://example.provider.host.com
i bought a domain and used CNAME to direct to
http://example.provider.host.com Now i bought a second domain and for SEO
reasons redirect from the the first domain to the second one. i do this in
urlFilter.groovy.
if(request.getRequestURL().find('a.com'){
redirect(url:h....b.com..., permanent:true)
}
That all works but now i get a lot of stacktrace errors about :
ERROR i18n.ParamsAwareLocaleChangeInterceptor - Error intercepting
locale change: Locale part "robots.txt" contains invalid characters
java.lang.IllegalArgumentException: Locale part "robots.txt" contains
invalid characters at
grails.plugin.cache.web.filter.PageFragmentCachingFilter.doFilter(PageFragmentCachingFilter.java:195)
at
grails.plugin.cache.web.filter.AbstractFilter.doFilter(AbstractFilter.java:63)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
what is that about?