I think I've found a bug, but not sure whether it's back-end or front-end related.
If MS SQL data files (.mdf and .ldf) are in the root directory of a drive (e.g. E:), backing up the databases by selecting the relevant db in the "Windows components to back up" in the GUI fails.
In our example, the error message returned was :
2017-07-13 10:51:56(error): Path "e:" does not start with volume "e:\"
2017-07-13 10:51:56(error): Error indexing files 0 of component "gh_test" of writer "SqlServerWriter".
If the MS SQL databases are stored in a directory, e.g. E:\Data, the backup work perfectly.
Backend server - Ubuntu Linux 14 LTS, UrBackup Server version 184.108.40.2065
Client - Windows Server 2008 R2 Enterprise, 64 bit; UrBackup Client version 2.1.6; MS SQL Server 2008 64 bit (10.0.5500)