If it works with one theme and not another, the issue is the theme causing the problem; so good to reach out to them about a fix as they may have some function not set properly breaking the sitemap, or they may be setting some kind of redirect to a custom "site map" post page index.
You can try this:
a) resave permalinks
b) clear transients using wp-optimize
However, it's very telling that if the sitemap works using default theme, and then you switch on another theme and it breaks Yoast -- most likely conflict with something in that theme !
