I have seen this when the 32-bit NetBackup client binaries was installed.
Have you confirmed that NB_51_4_M_280950.winnt.x64.exe (base x64 client software) is installed on your x64 clients?
Extract from README:
Windows 2003 x64 is not supported on the base CD-ROM version of NetBackup 5.1.
This is a new recompile of NetBackup software for 64-bit architecture.
Therefore, this separate NetBackup 5.1 MP4 container must be applied.