Just reporting that, following some hint on the net, I naively tried to add the file ISRG_Root_X1.crt to /etc/ssl/certs/ on the PB, but that didn't help, meaning that I still get "CURL returned 60" when trying to open an acsm.
To copy the certificate itself on the (rooted) PB, I sftp'd it, after having remounted /ebrmain as read-write
Code:
mount /ebrmain -orw,remount
The actual location of the other certificate files is a bit convolved: /etc/ssl/certs files are links to ../../../cramfs/share/ssl/certs/, which ultimately translates into /ebrmain/cramfs/share/ssl/certs/. /usr/share/ssl/certs/ seems the same thing. Whatever.
I whishfully reverted
Code:
mount /ebrmain -oro,remount
to limit further potential damage.
The file ISRG_Root_X1.crt itself I had lying around on the laptop in some /usr/share/ca-certificates/mozilla folder. I checked by inspection that a couple of other certificates found both there and on the PB were identical, just to be sure.