Tiles errors in Struts versions > 6.3.0.2

Our web application based on Struts 2 + Tiles framework starts to give 
errors on some jsp pages if I try to use a newer Struts version than 
6.3.0.2.

Exactly the same codebase works fine on previous Struts versions (the 
only changes are in the pom file and concern the versions of 
struts2-core, struts2-tiles-plugin, struts2-config-browser-plugin and 
strutrs2-json-plugin).

On 6.6.0 the stacktrace has snippets like:

> Caused by: org.apache.tiles.request.render.CannotRenderException: 
> java.io.IOException: JSPException including path 'xyz.jsp'.
>     at 
> org.apache.tiles.core.impl.BasicTilesContainer.render(BasicTilesContainer.java:395)
> ...
> Caused by: java.io.IOException: java.io.IOException: JSPException 
> including path 'xyz.jsp'.
>     at 
> org.apache.tiles.request.servlet.ServletUtil.wrapServletException(ServletUtil.java:56)
>     at 
> org.apache.tiles.request.jsp.JspRequest.doInclude(JspRequest.java:118)
> ...
> Caused by: java.lang.NullPointerException
>     at 
> org.apache.struts2.components.IteratorComponent.start(IteratorComponent.java:309)
>     at 
> org.apache.struts2.views.jsp.ComponentTagSupport.doStartTag(ComponentTagSupport.java:51)
where xyz is a path to a jsp file of ours.

Curiously, Struts 6.4.0 stacktrace for the very same page gives a 
different error:

> HTTP ERROR 500 Cannot find definition named 'nodeGradesView.page'
> Caused by: org.apache.tiles.core.definition.NoSuchDefinitionException: 
> Cannot find definition named 'xyz.page'
>     at 
> org.apache.tiles.core.impl.mgmt.CachingTilesContainer.render(CachingTilesContainer.java:108)
> ...

where xyz.page is a tiles definition of the view that xyz.jsp would 
belong to.

These errors are a bit cumbersome and have left as stuck at Struts 
version 6.3.0.2.

Best regards

Heikki