Goran Sarenkapa I believe the issue here is how you are configuring things.
The select CSS style is used to trigger the AJAX field population and the intent is that the style should only be present on select tags generated by f:select.
Your plugin is using a very non-Jenkins way to build its configuration, in fact you are not even using the AJAX select population that is triggered by adding the select CSS style: https://github.com/jenkinsci/zap-plugin/blob/development/src/main/resources/org/jenkinsci/plugins/zap/ZAPDriver/config.jelly#L246
I suspect that your plugin has always been making 404 requests and getting 404 responses but this change is now just showing the error, have a look at the network request inspector when loading the config page to see if my theory is correct.
There are two ways to your issue:
1. Just remove the select CSS class from your <select> tags; or
2. Switch to the Jenkins idiomatic AJAX select population by adding a doFillSelectedExportFormats() method to your descriptor and either provide the required fillUrl attribute in your <select> tag or switch to using <f:select>