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:
Download the new LetsEncrypt Root:
ISRG Root X1
Download the new LetsEncrypt Intermediate:
New R3 Intermediate