I have posted this with k2store devs and sh404sef: anything-digital.com/forum/extension/sh404sef/9762-k2store-and-sh404sef-seems-to-conflict.html?p=1#p58542

I am asking here if there are any ideas on if k2's router is conflicting with k2stores ajax mini-cart on pages with only 1 child category. The mini-cart works fine on pages with at least 2 child-categories.

It does not work on this page:
www.mygardenhobby.com/packaging-containers/cases.html
The issue may have to do with when Add to cart is clicked, the server is returning a 301 response.

But it will work if there is one more nested k2 category like this page:
www.mygardenhobby.com/packaging-containers/cases/15450-roto-molded-series.html

I have set the sh404sef / by component for K2store: to "Use joomla router" and "leave as non-sef", but neither resolved the issue.

Hi. Since K2Store is a different component Joomla! will trigger it's own router when generating the links. K2 router cannot affect links to third-party extensions ( like com_k2store ). This looks like an issue of K2Store.

Thanks for the hack described here:
getk2.org/community/English-K2-Community/147293-K2-Categories-URL-SH404SEF

This topic is not "solved" but at least there is a workaround.

You are welcome.