"error while loading shared libraries: libnv6json.so: cannot open shared object file: No such fil (4046642)

Customer tried to upgrade existing NetVault Backup Server to 8.6.1 (from 8.5.3) on Debian Lenny x86 and received the following error message:

Performing upgrade tasks
Upgrading configuration
/usr/netvault/bin/nvconfigupdate: error while loading shared libraries: libnv6json.so: cannot open shared object file:

No such file or directory
Failed to update configuration settings
Post install script failed!

I performed some tests and mimicked the customers installation i.e. installed NVBU 8.5.3 (netvault-R2010FEB03-HALO-LinuxX86-64.tar) 64Bit, making sure that all the libraries were installed:

1. apt-get install ia32-libs lib32nss-mdns
2. ./install
3. ldconfig
4. rm -rf /usr/netvault
5. ./install

This was successful and tested backups.
Then installed NVBU 8.6.1 (netvault-R2011OCT12-JUNO-LinuxX86Hybrid.tar) as an upgrade and failed with the same error as before.

I then uninstalled the whole installation and installed the NVBU 8.5.3 from scratch as above and then followed section 4.10.1 of the NVBU Installation Guide, as this was the missing step:

https://support.quest.com/Search/SearchDownloads.aspx?dsNav=Nea%3aTrue%2cN%3a4294957250-268446836-35%2cNr%3aAND%28AND%28AND%28OR%2855%2c54%2c56%2c57%2c35%2c59%29%2cP_SecurityRole%3aEXTERNAL%29%2cP_SecurityRole%3aEXTERNAL%29%2cP_SecurityRole%3aEXTERNAL%29&ActiveTab=1

Performed backups and completed.
Lastly installed NVBU 8.6.1 as an upgrade and installed successfully.
Performed backups again and completed too.