In brXM 15.2.1 in Experience Manager section, Sitemap section is not loading and customer is getting error. This worked well in 15.1.4

Findings

It turns out that it only happens during first opening of a channel. For some reason, only during first opening of the channel, the HST redirect to the SPA including

/root

in the pathInfo.

Then the SPA loads fine as the page delivery api (PDA) gets fetched with /root in the path, which maps to the sitemap item for the homepage returning the 'home page' PDA.
However, the HTML comments include then Hst-Path-Info equal to '/root', but for '/root', there is no URL item as it is '/'

Why the redirect is to '/root' is yet unclear, but for now we can already make sure the BE is resilient for a PDA request to '/root' (or '/foo' is mount indicates 'foo' is the homepage) by using '/' in the HST comment then