Debian of newest versions changes the algorythms how to sign packages and repositories. RTM deb-package is working, but the repository it sets (https://www.rememberthemilk.com/download/linux/debian jessie InRelease) is outdated and doesn't support modern mechanisms.
Please fix it.

Current output on Debian Trixie (Linux Mint LDME7) is:

Get:9 https://www.rememberthemilk.com/download/linux/debian jessie InRelease [5966 B]
Err:9 https://www.rememberthemilk.com/download/linux/debian jessie InRelease
Sub-process /usr/bin/sqv returned an error code (1), error message is: Verifying signature: Malformed MPI: leading bit is not set: expected bit 8 to be set in 110 (6)
Hit:11 https://www.synaptics.com/sites/default/files/Ubuntu stable InRelease
Warning: OpenPGP signature verification failed: https://www.rememberthemilk.com/download/linux/debian jessie InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Verifying signature: Malformed MPI: leading bit is not set: expected bit 8 to be set in 110 (6)
Error: The repository 'https://www.rememberthemilk.com/download/linux/debian jessie InRelease' is not signed.
Notice: Updating from such a repository can't be done securely, and is therefore disabled by default.
Notice: See apt-secure(8) manpage for repository creation and user configuration details.