For me it happens when you *do not* enter a value in a field that maps to an integer in your action.
The jira, located at: https://issues.apache.org/struts/browse/WW-2971, says that it has been resolved, but still the problem remains. This particular incident mentions the conversion from a double, but it also is happening with an int.
Does anyone have any suggestions as to how to fix this?
I don't know if this will help, but here is a list of my jars: