I was with this problem too , but I think I solved.
In sitemap-core.php on line 2181 I replaced $this->RemoteOpen for the function file_get_contents
when occurr an error, Google returns a page with “Error 400”, then on line 2183 i just replaced
if($pingres==NULL || $pingres===false) {
for this:
if($pingres==NULL || $pingres===false || strpos($pingres,”Error”)!==false) {
Well, this works for me.