Today I got the information, that my certificate will expire in 20 days. I am confused because since the beginning in 2020 it had been automatical renewed. I never got a notice by Let's Encrypt indicating malfunction occurs.
What should I do now?
Hi @Sascha1
please read the complete mail and the link shared in the mail, not only a small part.
The link about mails has the complete solution.
Hallo Sascha1,
auf DSM anmelden->Systemsteuerung->Sicherheit->Zertifikat->HinzufĂĽgen->Zertifikat erneuern
Welcome to the Let's Encrypt Community 
To supplement @JuergenAuer's recommendation: