Finally, with kind assistance of the nice people in this community, the solution for Debian has been found.
My Debian's CA store didn't consist ISRG Root X1 root neither, of course, ISRG X2 nor E1 certificates. And also I found that it doesn't accept chained certificates, like 0000_chain.pem. So:
- Create 3 separate ISRG1.crt, ISRG2.crt and E1.crt in pem-format.
- Copy them into /usr/local/share/ca-certificates/
- update-ca-certificates