Ok, I have found a solution.
The problem is due to the fact that the KeePassRPC.plgx fails to compile on Linux Mint 19.1 due to a Mono problem. Here are the steps to solve:
-
Upgrade Mono to the latest available version: go here -> https://www.mono-project.com/download/stable/
and follow the instructions for Ubuntu 18.04 -
Update the repositories and upgrade mono:
sudo apt update
sudo apt upgrade (this will upgrade all the packages that are marked as upgradeable, if you don’t want to upgrade everything you’ll need to update only those you want to) -
Install mono-devel:
sudo apt install mono-devel
Now the plugin works correctly