I'm experiences the same kind of problems.
In our STRUTS web application, we're using the netspade-compression-filter-1.0.jar. Running under Websphere Application Server 5.1, this works fine.
We've now converted our application to Websphere Application Server 6.1 (compiled in JDK 1.5).
The compression filter is not working correctly.
Directly calling a jsp-file fails (only part of the HTML is produced till the end </HEAD> tag).
The index.jsp (accessed when going into the application using the URL/contextroot, forwards to another jsp. This jsp is shown correctly, but when using forwards to other STRUTS actions, an HTTP 500 occurs.
Anybody had these problems before and solved them?
Netspade seems to have died out, so I will try the GZIPFilter in the jspbook.jar and see if this will solve the issue.