Fetch request to backend within same domain fails net::ERR_CERT_AUTHORITY_INVALID
Solution 1:
Hard to tell, but check that the site is properly setup for both IPv4 and IPv6 in case there are DNS entries for both IPv4 and IPv6. It is a common problem that a site is only properly setup for IPv4 and this seems to work from everywhere testet - but fails to work for networks which actually have IPv6 connectivity (i.e. local network, often mobile networks).
Additionally check that the results from a DNS lookup for both IPv4 and IPv6 are the same from all places and that there are no local hosts files which might have conflicting settings.