It could be related to log4j.
Do you have log4j.jar file in the websphere java classpath (as defined in the startup file) as well as the application classpath ?
If you do make sure that the log4j.jar file is in the java classpath and that it is NOT in the web-inf/lib directory of your webapp.
It can also be related with the ant version (may be not your case, but I do put it here for reference):
You have a .class file in your class path (i.e. not a directory or a .jar file). Starting with ant 1.6, ant will open the files in the classpath checking for manifest entries. This attempted opening will fail with the error "java.util.zip.ZipException"
The problem does not exist with ant 1.5 as it does not try to open the files. - so make sure that your classpath's do not contain .class files.
On a side note, did you consider having separate jars ?
You could in the manifest of your main jar, refer to the other jars with this attribute:
Class-Path: one.jar two.jar three.jar
Then, place all of your jars in the same folder.
Again, may be not valid for your case, but still there for reference.
org.apache.catalina.startup.TldConfig tldScanJar
WARNING: Failed to process JAR [jar:../opensaml.jar!/] for TLD filesZipException
to resolve this problem add opensaml.~.jar into Application lib folder. – Ballance