The LetsEncrypt Root Certificate (DST Root CA X3) will expire on 30-Sep-2021. While there is a new Root Certificate available for a while, some devices and browsers might show an error after that date as they might not have got the new Root Certificate.

Here is the simplified overview:

Old LetsEncrypt chain:

DST Root CA X3 : Expires 30-Sep-2021
R3 (Intermediate): Expires 29-Sep-2021

New LetsEncrypt chain:

ISRG Root X1 : Expires 04-Jun 2035
R3 (Intermediate): Expires 15-Sep 2025

If you encounter a problem after 30-Sep-2021 you can just install the new Root Certificate into the Trusted CA Store of the device and you should be good.

For a more detailed analysis, go over to Scott Helme's blog:

https://scotthelme.co.uk/lets-encrypt-old-root-expiration/

Download the new LetsEncrypt Root:
ISRG Root X1

Download the new LetsEncrypt Intermediate:
New R3 Intermediate