[Solved] Template "modular/hero.txt.twig" is not defined

I've created a new theme inherited from Grav, my modular landing page fails with:
[code]An exception has been thrown during the rendering of a template ("Template "modular/hero.txt.twig" is not defined.").[/code]
Why on earth is it looking for ".txt" while processing modular.html.twig???
This only seems to happen in IE11, all other browsers give the correct page.

Any suggestions where to start looking?

Thanks.

Solved: for some reason the order of page types in system.yaml turned out to be important, possibly just for modular pages and IE11 as client. The install puts "txt" first, moving this (and "xml") below "htm/html" solved the problem.