I found the root cause. Nothing to do with wildcard mapping. The issue is bound to the pre 2.5.2 way getText(key) for any class extending ActionSupport. In this project, in contrast to other more complex ones, this worked out until 126.96.36.199:
The main action class MyClassDoingNothing extending ActionSupport does several things. It holds a reference to a Utility class also extending ActionSupport, which does fill in Lists of other UtilityClasses partially also extending ActionSupport. I did this to be able to call getText(key) in some validation context. Now this seems to set container to null after 2.5.12 leading to a NP.
As soon as I replace the Struts method getText(key) with my custom wrapper Texti18n.getText(key) and remove the extends ActionSupport in the Utility classes, the NP is gone.
Now obviously I did use ActionSupport not in the correct context. I'd still vote for a handy way to use a something like getText(key) as Struts method thorugh any point in a project.