I think I have my answer (putting it here for anyone else using X9)... I must have been coming from 3.55 or older. It seems later firmware lack BOTH Windows AND Linux support for 64-bit (x86_64). I have asked Supermicro why on earth they've done this (and left support for macOS 64-bit working?)
user@pc:~$ strings SMT_X9_355.bin |grep -i pack.gz
SOL__V0.5.11.jar.pack.gz
iKVM__V1.69.21.0x0.jar.pack.gz
liblinux_x86_64__V1.0.5.jar.pack.gz
liblinux_x86__V1.0.5.jar.pack.gz
libmac_x86_64__V1.0.5.jar.pack.gz
libsun_SPARC__V1.0.5.jar.pack.gz
libwin_x86_64__V1.0.5.jar.pack.gz
libwin_x86__V1.0.5.jar.pack.gz
user@pc:~$ strings SMT_X9_357.bin |grep -i pack.gz
SOL__V0.5.11.jar.pack.gz
iKVM__V1.69.38.0x0.jar.pack.gz
liblinux_x86__V1.0.5.jar.pack.gz
libmac_x86_64__V1.0.5.jar.pack.gz
libwin_x86__V1.0.5.jar.pack.gz
user@pc:~$ strings SMT_X9_362.bin |grep -i pack.gz
SOL__V0.5.15.jar.pack.gz
iKVM__V1.69.39.0x0.jar.pack.gz
liblinux_x86__V1.0.5.jar.pack.gz
libmac_x86_64__V1.0.5.jar.pack.gz
libwin_x86__V1.0.5.jar.pack.gz
user@pc:~$ strings SMT_X9_364.bin |grep -i pack.gz
SOL__V0.5.15.jar.pack.gz
iKVM__V1.69.39.0x0.jar.pack.gz
liblinux_x86__V1.0.5.jar.pack.gz
libmac_x86_64__V1.0.5.jar.pack.gz
libwin_x86__V1.0.5.jar.pack.gz
user@pc:~$ strings SMT_X9_366.bin |grep -i pack.gz
SOL__V0.5.17.jar.pack.gz
iKVM__V1.69.41.0x0.jar.pack.gz
liblinux_x86__V1.0.7.jar.pack.gz
libmac_x86_64__V1.0.7.jar.pack.gz
libwin_x86__V1.0.7.jar.pack.gz
I've just realised something else too.
I think IPMIView stores a cache of the libraries it gets from previous connections.
So I probably was on firmware 3.64 with IPMIView 2.21.1 perfectly working earlier this year. But that is likely only because previously I had used the same copy of IPMIView to connect to the same server when it was running 3.55 or older. This would have cached liblinux_x86_64__V1.0.5.jar which is compatible with the x86_64 JRE bundled with IPMIView (all versions that I've ever used).
However
The 3.66 version hands out a JNLP file which references liblinux_x86_64__V1.0.7.jar (different version number). So this file is not cached by existing IPMIView installations which has previously cached liblinux_x86_64__V1.0.5.jar.
This explains why my IPMIView 2.21.1 was working with 3.64... despite 3.64 also lacking Windows & Linux 64-bit libraries in the firmware.
So this workaround no longer works...
Frequently Asked Questions
www.supermicro.com
Also this FAQ...
Frequently Asked Questions
www.supermicro.com
Where it says " If you connect to only ATEN and AMI X9 firmware, 64 bit JRE should be workable (for KVM). "
Is completely false now... and has been for anyone using a freshly installed copy of IPMIView on a firmware newer than 3.55.
I've raised this with Supermicro but all I've got back is... "I will forward your feedback, but as i mentioned, the firmware fix works for the vast majority of users still using this old platform. Due to its age it's highly unlikely further updates are provided."