Thank you for your message which led me to this page:
In this page they explain that the DST Root CA X3 will expire on September 30, 2021 which may explain the problem I am having, because we imported the following intermediate certificates into forti web:
What should I do ?
Acoording to Nummer378 in this post my site should be serving two intermediates (R3 signed by ISRG Root X1 and ISRG Root X1 signed by DST Root CA X3)
With what should I replace DST Root CA X3 ?