That's a very good question!
Let's Encrypt recently changed their issuance chain. Since 4th of May the default chain is:
End-entity certificate ← R3 ← ISRG Root X1 ← DST Root CA X3
It seems your webserver is missing one of the intermediate certificates: it's sending the ISRG Root X1 signed by DST Root CA X3, but it's lacking the intermediate R3 signed by ISRG Root X1.
Maybe you should contact the support of your ISP manager panel?