Problem with gnus on emacs-26 on debian 10: "Mail source (file) error (Searching for Program)"

[Steinar Bang <sb@dod.no>]

Platform: Debian 10.0 "buster", amd64
	GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4) of 2019-02-03, modified by Debian
	Gnus v5.13

When starting gnus and when fetching new mail and news with 'g', I get
the following error message in the minibuffer:
 Mail source (file) error (Searching for Program)

According to the docs[1]:
 You tell Gnus how to fetch mail by setting mail-sources (see Fetching Mail) to a mail source specifier.

Doing 'C-h v mail-sources RET':
 mail-sources is a variable defined in ‘mail-source.el’.
 Its value is ((file))

Also according to [1], about "file":
 :path
   The file name. Defaults to the value of the MAIL environment variable or the value of rmail-spool-directory (usually something like /usr/mail/spool/user-name). 

Doing 'C-h v rmail-spool-directory RET
 rmail-spool-directory is a variable defined in ‘rmail.el’.
 Its value is "/var/mail/"

which points to the directory where my inbox resides:
 sb@lorenzo:~$ ls -al /var/mail/sb 
 -rw-rw---- 1 sb mail 28529 Aug  4 10:39 /var/mail/sb
 sb@lorenzo:~$ 

The MAIL environment variable also points to my inbox:
 sb@lorenzo:~$ env | grep MAIL
 MAIL=/var/mail/sb
 sb@lorenzo:~$

Does anybody know what the problem might be?

Thanks!


- Steinar


References:
[1] <https://www.gnu.org/software/emacs/manual/html_node/gnus/Mail-Source-Specifiers.html>