I have installed digikam on Tumbleweed GNOME. When I start digikam, I get the
message: "Error while opening the database. digiKam will try to automatically
reconnect to the database." digikam stalls at this point.

digikam uses either sqlite (default) or MySql to create a number of databases.
The problem with the current digikam package is that it loads the appropriate
driver for MySql, but not for sqlite. The missing driver is in the package
libQt5Sql5-sqlite

This problem has been around for some time, affecting the Leap distros as well
as the current openSUSE 15 in addition to Tumbleweed.

Please update the digikam package to make sure that libQt4Sql5-sqlite is
installed.