Since Let's Encrypt changed their API from v01 to v02 it isn't possible to request certificates when using DSM 6.1.X.
Workaround: exchange the syno-letsencrypt module with a version from a DSM 6.2.X release.
Preparations:
enable SSH access on your box
download this file (from an original DS920+, DSM 6.2.3-25426 Update 3), extract it and copy it to a shared folder of your NAS: https://mega.nz/file/889yCJ5S#Y3UtxPe_qQfRkpxQeALzsFEAIx_NDxLVx2FPLu9HT_I
After you have downloaded the file mentioned open a SSH connection to your NAS
Backup your current syno-letsencrypt module by
sudo cp /usr/syno/sbin/syno-letsencrypt usr/syno/sbin/syno-letsencrypt.bak
Copy the new syno-letsencrypt from your shared folder (for example: volume1/Downloads) to the sbin folder:
sudo cp /volume1/Downloads/syno-letsencrypt /usr/syno/sbin/
Modify the file to execute it:
sudo chmod 755 /usr/syno/sbin/syno-letsencrypt
Open the letsencrypt.default with vi and modify it:
sudo vi /usr/syno/etc.defaults/letsencrypt/letsencrypt.default
Search for the string "https://acme-v01.api.letsencrypt.org/directory"
Press "i" and change the 01 to 02 in this string (should be "https://acme-v02.api.letsencrypt.org/directory")
To save and exit, press ESC, type :wq (ENTER)
Reboot your NAS and Let's Encrypt should work again