I support an existing application and we've been working to upgrade to struts 1.3.10. This app has been running several years. I had never seen this warning message until the upgrade attempt, and it's only appearing for a few pages. Due to multiple issues, we've trying to run down all possibilities and this warning was one of them. Here's what I found to be causing this warning.
Most of the suggestions I had found talked about ensuring that all Forms that required validation be included in the validation.xml file as a <form> element, even if it is jsut an empty element. If the relevant <form> element can't be located, you get the warning. Well, my validation.xml had all the relevant <form> entries, so the solution is the same idea, but instead of the <form element not being there, the validation was looking for it using the wrong key value for the couple pages where I had the warning.
Most of the app's Form classes (myForm.java) are extending org.apache.struts.validator.ValidatorForm, but a couple (the ones generating the warning) extend org.apache.struts.validator.ValidatorActionForm (which itself extends ValidatorForm). In reviewing the API docs for both classes, there is a subtle difference in the key value that is retrieved from the <action> element of struts-config.xml. One class retrieves the 'path' attribute, the other the 'name' attribute. This key value is then used to locate the relevant <form> element in validation.xml, per the element's 'name' attribute.
Once I changed the couple Form classes to use ValidatorForm, the proper key value was retrieved which matched to the entries in validation.xml and the warning message went away.
For reference...
http://struts.apache.org/release/1.2.x/api/org/apache/struts/validator/ValidatorForm.html
https://struts.apache.org/release/1.3.x/struts-extras/apidocs/org/apache/struts/validator/ValidatorActionForm.html