Can't backup sources.list

[Aclass]

Hi!!

I'm trying to install Plesk 8.0 under Ubuntu 5.10 and always I get the same error when I go to the Next page after select the components I want to install. See the report:

Installation type
===============================================================================

Please select the components you wish to install:

1. [*] Base packages of Plesk
2. [*] Plesk Updater
3. [*] Frontpage 2002 support
4. [!] Apache ASP support
5. [*] Mailman mailing list manager support
6. [*] PosgreSQL server support
7. [*] Tomcat Java Servlets support
8. [*] SpamAssassin support
9. [*] SPAM blocker for QMail daemon
10. [*] Apache mod_python module
11. [*] Application vault packages
12. [*] Additional Horde (webmail) components for Plesk
13. [*] Additional Plesk manuals
14. [*] Plesk Backup Manager
15. [*] Dr. Web antivirus
16. [*] Plesk Professional Web Site Editor
17. [*] Plesk API [former Plesk Agent]
18. [*] SSHTerm - SSH Terminal java applet
19. [*] Plesk migration manager
20. [*] Plesk Firewall module
21. [ ] Plesk Counter-Strike game server module
22. [*] Plesk VPN module
23. [ ] Plesk Battlefield 1942 game server module
24. [ ] Plesk Battlefield2 game server module
25. [*] Plesk Fileserver module and SMB file server package
26. [*] Watchdog (System monitoring module)
27. [ ] SiteBuilder and Sitebuilder module for Plesk
28. [ ] SiteBuilder publishing support for Plesk
29. [*] ColdFusion support for Plesk
30. [*] Acronis TrueImage module

N) Next page; P) Go back; Q) Cancel installing;
A) Select all; D) Deselect all;
To toggle component enter its number;
Type a number or a character of desired action [N]:
resolve packages with apt-get
ERROR: Can't backup sources.list
ERROR: Installation failed
aclass@acp72:~$

Thanks a lot for any help.

Alfonso