Following a fresh install or upgrade to Server Backup Manager version 6.18 or newer, the following error may be observed in the server.log file.
The SBM was updated to use OpenJDK 8 in version 6.18 and newer. For a number of years, OpenJDK has not bundled fonts or the ability to discover and use them. The Backup Manager dashboard makes use of fonts that are not natively included. The Dashboard widgets display properly, however the aforementioned error is still logged.
Additional packages will be required in order to add fonts support and silence the errors in the server.log file.