The Openmediavault-Plugin cannot be instaled because it has an explicit dependendy on libcrypto++9, which is not available on Debian stretch. The dependency should be removed, because urbackup already has a dependency on libcrypto (libcrypto++9 in an old version an libcrypto++6 in the latest version)
Idk who builds that package. The directly linked Debian packages on the download page are compiled without crypto++ dependency and don’t have those poblems because of that.