Once again I encountered an issue installing a .deb package on Ubuntu 18.04. First I assumed that package must be broken, but when

… I realized that the Ubuntu updater too failed, I had a feeling that this might be related.

First: the Error of the deb-package installation⌗

sudo dpkg -i boostnote_0.11.17_amd64.deb
[sudo] password for tokosh:
dpkg-deb: error: 'boostnote_0.11.17_amd64.deb' is not a Debian format archive
dpkg: error processing archive boostnote_0.11.17_amd64.deb (--install):
 dpkg-deb --control subprocess returned error exit status 2
Errors were encountered while processing:
 boostnote_0.11.17_amd64.deb

Second: Sync the package index files + Solve the Issue⌗

sudo apt-get update
...
Fetched 1,857 kB in 4s (452 kB/s)
Reading package lists... Done
W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list.d/google-chrome-unstable.list:3 and /etc/apt/sources.list.d/google-chrome.list:3
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list.d/google-chrome-unstable.list:3 and /etc/apt/sources.list.d/google-chrome.list:3
...

Strangely that used to work before. But well, the content of google-chrome.list and google-chrome-unstable.list are identical. So, after commenting out the source-line in one of them, the apt-get update succeeded.

After that the above .deb-package installed successfully.