HTTP listeners stop responding

Hi,

we are using Glassfish Enterprise Server v2.1 for our SOAP web services.
Every once in a while some or all of the HTTP listeners stop responding (the admin listener on port 4848 never seems to cause problems). This causes the web service clients to receive time out errors.

When I check Glassfish's logs, I find the following error.

Restarting Glassfish fixes this issue.

Any ideas as to what might cause this behaviour and how it could be fixed?