I just went through a nightmare of an upgrade from an old mautic instance 2.15.3 to 2.16.3 and came accross a number of problems along the way that maybe helpful to someone some day.
So as follows:
Started upgrade via GUI - it just hung
Went for command line upgrade and received this error:
Compile Error: require_once(): Failed opening required ‘PHPEXCEL_ROOTPHPExcel/Shared/PCLZip/pclzip.lib.php’ (include_path=’.:/usr/share/php’)
Found this article to fix the error: https://kirelos.com/install-mautic-self-hosted-email-marketing-platform-on-ubuntu-18-04-server/
Re ran upgrade and ran into this error
Attempted to load class “ZipArchive” from the global namespace.
Did you forget a “use” statement for “PHPExcel_Shared_ZipArchive”?
Followed this tutorial: https://mauteam.org/mautic/mautic-admins/mautic-update-how-to-dont-click-that-button/
Alas - still did not help, and the dreaded site offline was still there.
Then found that I needed to install php-zip which I did and then everything ran well!!
Hope this helps.