Thank you mnordhoff,
I’ve found 2 entries in the cacert.pem file which correspond to ‘IdenTrust’, I found 1 entry which correspond to ‘ISRG’, no entry specific to Let’s Encrypt.
I shall try again to use this cacert.pem fil for authentication and come back with the results.
Apart from that the server is presumably correctly configured as it works for 90% of our park (SSL connexion problems with only 10% of clients)