I run debian stable (bullseye) and I use the official R backports. See

https://cloud.r-project.org/bin/linux/debian/

I added a line to mu repositories

$ cat /etc/apt/sources.list | grep r-project
deb http://cloud.r-project.org/bin/linux/debian bullseye-cran40/

And everything has been good until today. Now, when I update, I get an error about the signature of the R repository when I run sudo apt update, see

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cloud.r-project.org/bin/linux/debian bullseye-cran40/ InRelease: The following signatures were invalid: EXPKEYSIG FCAE2A0E115C3D8A Johannes Ranke (Wissenschaftlicher Berater) <johannes.ranke@jrwb.de>
W: Failed to fetch http://cloud.r-project.org/bin/linux/debian/bullseye-cran40/InRelease  The following signatures were invalid: EXPKEYSIG FCAE2A0E115C3D8A Johannes Ranke (Wissenschaftlicher Berater) <johannes.ranke@jrwb.de>
W: Some index files failed to download. They have been ignored, or old ones used instead.

The signature has always been working until now. I reimported it following the instructions here

https://cloud.r-project.org/bin/linux/debian/#administration-and-maintenance

which I paste below

You can fetch and import the current key using

apt-key adv --keyserver keyserver.ubuntu.com --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF'

Note that you need to add sudo if you are not running this as root.

If this fails, it may be due to a firewall blocking port 11371. In this case, you can search for 0xE19F5F87128899B192B1A2C2AD5F960A256A04AF at https://keyserver.ubuntu.com, and copy the key block shown when klicking on the link in the line starting with pub into a plain text file, named, for instance, jranke.asc which you add to apt with (sudo) apt-key add jranke.asc.

but it does not work. I import the signature, but when I update I am always told it is not valid. Anyone else is experiencing the same? Any ideas? Thanks!