It is possible you changed something in your list of repositories or it may even occur from a corrupted file. When you run sudo apt-get update (or use the Refresh button in a package manager), a list of packages will get downloaded from the Ubuntu servers. These files are then stored in /var/lib/apt/lists/. If I'm ever having issues running update my last ...
This is a typical behavior I have seen with a "weak" SD Card. It sometimes works or not maybe depending on different hardware because of very small different electrical characteristics. You wrote that you always use the same SD Card. I suggest to try with another SD Card to exclude that's not the reason.
I have been 3 days working over my email sending routines...
I am using Lazarus 1.6 with Freepascal 3.0... and Raspbian Stretch
I am using xmailer as it has an easy component to send emails.
To make it work I did the following:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install libssl1.0-dev