I went to the download page of the current version and just below the button to download the most recent version there is a line that reads"
You can also download [ eM Client 7 ] or check the release history for other releases.
click on the text to download for eM7 most recent version. Install the most recent version of 7 which will allow the calendar to work properly. Verify everything in version (https://www.emclient.com/dist/v7.2.37929/setup.msi) is working with your calendar, than download the latest version of eM Client 8 and install. This fixed my issue as I had everything working fine on my old computer which started with version 7 and installed version 8 without uninstalling version 7, but wouldn’t work on my new computer when I just installed version 8 or when I tried to install version 7.0 and then version 8. You must install the version of 7 shown above for it to work with your version 8 install. When I did this procedure with my new computer, everything worked well.
Hope this works for you.