I'm trying to run a tar backup using flexbackup.

DU and DF report about 116G of data.

The tape drive is an LTO-2 drive (tandberg) with 200G uncompressed capacity. The tape has been rewound and erased.

The backup keeps failing with "device out of space" errors. At one point I saw an error where the size of the backup was claimed to be over 215 GB.

I've tried different block sizes and seem to keep getting
the same capacity errors.

The "flexbackup -test-new-tape" declares success.

The backups were working when the machine was configured to run using an older version of Centos. I've since upgraded to Debian 9 (stretch). Since the upgrade I seem to be having issues.

Ideas?

Perhaps there are many hard links. These normally don't show up in du and df. However, you can get du to count them. From man du:
Try that and see.

Is there a way for your backup software to store hard links as links, not copies?

Edit: I noticed you are using tar. After reviewing the manual, I tried tar cf on some hardlinked files, and I see that it stores links, not copies. If I add the --hard-dereference option, then it stores copies. Is your tar command (or flexbackup) doing something like that? I don't have a tape drive, so I can't tell if that makes a difference.

And the same goes for symbolic links, they must not be DEreferenced either (or the file will be stored twice on the tape).
Finally: are there many compressed files in the data to be backup'ed (.gz, .bz2, .xz, .zip, .rar, mp3 or mp4, etc. files)?
If so the hardware compression OF the tape drive will work against you, because trying to compress already compressed data will make it LARGER, not smaller.
I explicitly switch compression ON or OFF with the command:
in my backup scripts ($COMPRESS is either 0 or 1, see man mt-st).
Depending on the data, of course.