The issues is with the letter version of libssl1.1 (= 1.1.1f-1ubuntu2) that needs to be downgraded. It was simple to do it with synaptic
instead of apt install
as follows:
sudo aptitude install libssl-dev
he following NEW packages will be installed:
libssl-dev{b}
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 1.582 kB of archives. After unpacking 8.005 kB will be used.
The following packages have unmet dependencies:
libssl-dev : Depends: libssl1.1 (= 1.1.1f-1ubuntu2) but 1.1.1g-1+ubuntu18.04.1+deb.sury.org+1 is installed
The following actions will resolve these dependencies:
Keep the following packages at their current version:
1) libssl-dev [Not Installed]
Accept this solution? [Y/n/q/?] n
The following actions will resolve these dependencies:
Downgrade the following packages:
1) libssl1.1 [1.1.1g-1+ubuntu18.04.1+deb.sury.org+1 (now) -> 1.1.1f-1ubuntu2 (focal)]
Accept this solution? [Y/n/q/?] y
The following packages will be DOWNGRADED:
libssl1.1
The following NEW packages will be installed:
libssl-dev
0 packages upgraded, 1 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 2.900 kB of archives. After unpacking 8.087 kB will be used.
Do you want to continue? [Y/n/?] Y
Basically don't accept the proposed solution and carry on the a package downgrade