I recently again came across JAVA errors/exception referring to JSP's when starting the Mapviewer with Java exceptions.
The reason was, that the Mapviewer startup batchfile did not include the
SET JAVA= path.... and a JAVA runtime version cud not complie the respective JSP's.
I was able to access the Mapviewer ADMIN Page but some other pages seem to have gone (cud not be compiled properly during runtime)
To overcome this obstacle recommend to include the JAVA SET command as follows:
(my startup batchfile assigning 512 MB RAM to the OC4J(Mapviewer)
Good luck !