First attempt with the UrBackup Client 2.3.4, then I applied the latest updates from Apple today, and re-ran the install with the same result. macOS Mojave 10.14.5
I’m not a programmer (just a system admin) but does it not look like there are some file missing that the install process is calling, “./postinstall” ? 
2019-05-21 10:51:56+02 computer-name installd[395]: PackageKit: Install Failed: Error Domain=PKInstallErrorDomain Code=112 "An error occurred while running scripts
NSFilePath = “./postinstall”;
NSLocalizedDescription = “An error occurred while running scripts from the package \U201cfinal.pkg\U201d.”;
NSURL = “file:///var/folders/zt/32s5w0qx1435wn137pby5q0h0000gn/T/selfgz138425550/final.pkg#output2.pkg”;
PKInstallPackageIdentifier = “org.urbackup.client”;
install.log (13.0 KB)
system.log (1.7 KB)