I had the same problem (just wanted to retrieve my Contacts) and managed to get them. In the following I will explain how. This method probably works with backups from other mobiles made with Sony PC Companion too.
I got the same problem (also only fullbackupdata), after rooting my mobile and not thinking of the problem, that it might not be able to restore the contacts.
Here's what I found out: I couldn't find anything with Google, so I decided to just look at the data of the file. It starts with:
I did some Googeling again and found out, that it is the same file like an ICs created adb backup.
I also found a project to retrieve Data from backups: ht tp s: //github.c om/nelenkov/android-backup-extractor
HOW YOU CAN RESTORE YOUR DATA:
Get your Fullbackupdata-Contactfile.
1. First go to your backupfile: (C:\Users\YourUserName\Documents\Sony\Sony PC Companion\SomeNameDependsOnLanguage\
2. Make a Copy of your backup-File (crtl+c, crtl+v)
3. We want to change the file extension, so make sure you see it. If you don't see a .dbk at the end of your file, go in the explorer window in the left upper corner on organise -> Folder and search options. Change to view and then deselect "Hide extensions for known File types"
4. Rename (F2) the file to backup.zip
5. Open the zip with 7zip or something similar.
6. Navigate to Applications\com.sonyericsson.android.contactsimpo rt and get the fullbackupdata file which is lying in this folder.
7. Put this file on your Desktop.
8.Download this jar-File: ht tp: //ge.t t/64TJmne/v/0?c
9. Put the .jarFile also on your desktop.
10. Doubleclick the .jar-File. After a few seconds a new file with the name "restore.tar" should show up (If not check if your fullbackupdata-File really has the name "fullbackupdata" and is on the desktop)
11. Open the .tarfile with 7Zip
12. Navigate to restore.tar\apps\com.sonyericsson.android.contacts import\f\
13. Copy full_backup_vcard.vcf to the Desktop.
If everything worked you're finished, you can open this .vcf File via Windows-contacts -> import or put it directly on your phone and import it there (open Contacts. Settingsbutton -> impot)
If you don't trust my .jar file, you can also download the github-project follow their orders and compile it yourself.
I hope this helped also other people. It took me quite a while, so I thought I should share my solution.
Ps: Sorry for the spaces in the links, I'm not allowed to post any