Your site is not sending any intermediate certificate. It is just sending the leaf certificate:
# openssl s_client -connect elearning.univ-bejaia.dz:443
CONNECTED(00000184)
depth=0 CN = elearning.univ-bejaia.dz
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = elearning.univ-bejaia.dz
verify error:num=21:unable to verify the first certificate
verify return:1
depth=0 CN = elearning.univ-bejaia.dz
verify return:1
---
Certificate chain
0 s:CN = elearning.univ-bejaia.dz
i:C = US, O = Let's Encrypt, CN = R3
---
This also explains while it works on some systems while it doesn't work with others. Clients that have R3 cached (or can load it externally) will have no problems connecting to your site, other clients will show validation errors.
Your site should actually be serving two intermediates (R3 signed by ISRG Root X1 and ISRG Root X1 signed by DST Root CA X3), but I'm still not understanding why you're trying to configure intermediate certificates manually. Nothing in the guides I've found about FortiWeb (1, 2) implies that this is necessary.