[UPDATE 17.03.2020: Microsoft has released SQL Server 2019 CU3 with an Ubuntu 18.04 repository. See: https://techcommunity.microsoft.com/t5/sql-server/sql-server-2019-now-available-on-ubuntu-18-04-supported-on-sles/ba-p/1232210 . I hope this is now fully compatible without any ssl problems. Haven't tested it jet.]
Reverting to 14.0.3192.2-2 helps.
But it's possible to solve the problem also using the method indicated by Ola774, not only in case of upgrade from Ubuntu 16.04 to 18.04, but on every installation of SQL Server 2017 on Ubuntu 18.04.
It seems that Microsoft now in cu16 messed up with their own patch for the ssl-version problems applied in cu10 (https://techcommunity.microsoft.com/t5/SQL-Server/Installing-SQL-Server-2017-for-Linux-on-Ubuntu-18-04-LTS/ba-p/385983). But linking the ssl 1.0.0 libraries works.
So just do the following:
Stop SQL Server
sudo systemctl stop mssql-server
Open the editor for the service configuration by
sudo systemctl edit mssql-server
This will create an override for the original service config. It's correct that the override-file, or, more exactly "drop-in-file", is empty when used the first time.
In the editor, add the following lines to the file and save it:
[Service]
Environment="LD_LIBRARY_PATH=/opt/mssql/lib"
Create symbolic links to OpenSSL 1.0 for SQL Server to use:
sudo ln -s /usr/lib/x86_64-linux-gnu/libssl.so.1.0.0 /opt/mssql/lib/libssl.so
sudo ln -s /usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0 /opt/mssql/lib/libcrypto.so
Start SQL Server
sudo systemctl start mssql-server
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