The problem may be with your web.xml file. I had a similar problem with a portlet that worked fine under 6.1GA1, but not with 6.1GA2. When Liferay deploys portlets (and intercepts, etc.), it overwrites the file and others to add filters, libraries, etc. Thus, the error is probably not in the file that was originally created, but with what Liferay generated.
In my case, Liferay changed the first part of web.xml from
<?xml version="1.0" encoding="UTF-8"?> <web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
to something like
<?xml version=<filters>[other stuff]</filters> <web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
So, that was the modified first line that caused the problem. I had an XML comment using <!-- --> , and deleting these sections fixed the problem. But this is a mistake IMHO.