Hello,
Having just installed both krdc and krfb, I can connect alright from the client (krdc) to the server machine (after opening up ports 5900 and 5901 on the krfb-side).
Unfortunately I now only see a completely useless black window within the blue frame of the krdc window. It includes the KDE-logo (cogwheel etc.). Another try resulted in seeing the workspace that was active on the krfb-side, including the task/system bar. Selecting any different item DID get honored on the krfb-side, but nothing was updated/changed on the krdc-side. I connect via vnc://user@host:5900. I also tried port 5901. I disabled all workspace effects on the krfb-side -- no change!

My systems are (output of inxi -F):
krfb-side:
krfb-Version: 5.0, KDE Frameworks 5.28.0
System: Host: hsdesk Kernel: 4.9.0-4-amd64 x86_64 (64 bit) Console: tty 2
Distro: Debian GNU/Linux 9 (stretch)
Machine: Device: desktop System: LENOVO product: 099217G v: ThinkServer TS200V
Mobo: LENOVO model: N/A BIOS: LENOVO v: 5JKT39AUS date: 02/26/2010
CPU: Dual core Intel Core i5 650 (-HT-MCP-) cache: 4096 KB
clock speeds: max: 3193 MHz 1: 2261 MHz 2: 1197 MHz 3: 1197 MHz 4: 1197 MHz
Graphics: Card: Intel Core Processor Integrated Graphics Controller
Display Server: N/A drivers: modesetting (unloaded: fbdev,vesa)
tty size: 184x45 Advanced Data: N/A out of X
Audio: Card Intel 5 Series/3400 Series High Definition Audio driver: snd_hda_intel
Sound: Advanced Linux Sound Architecture v: k4.9.0-4-amd64
Network: Card: Intel 82578DM Gigabit Network Connection driver: e1000e
IF: eno1 state: up speed: 100 Mbps duplex: full mac: 70:f3:95:00:00:2d
...

krdc-side:
krdc-Version: 17.08.3
system: Host: hslaptop Kernel: 4.10.0-37-generic x86_64 (64 bit) Desktop: KDE Plasma 5.8.8
Distro: neon 16.04 xenial
Machine: System: Sony (portable) product: VGN-AW11M_H v: C6010CXT
Mobo: Sony model: VAIO Bios: American Megatrends v: R0200Y2 date: 08/12/2008
CPU: Dual core Intel Core2 Duo P8400 (-MCP-) cache: 3072 KB
clock speeds: max: 2267 MHz 1: 1600 MHz 2: 1600 MHz
Graphics: Card: NVIDIA G98M [GeForce 9300M GS]
Display Server: X.Org 1.19.3 drivers: nvidia (unloaded: fbdev,vesa,nouveau)
Resolution: 1680x945@59.95hz
GLX Renderer: GeForce 9300M GS/PCIe/SSE2 GLX Version: 3.3.0 NVIDIA 340.104
Audio: Card Intel 82801I (ICH9 Family) HD Audio Controller driver: snd_hda_intel
Sound: Advanced Linux Sound Architecture v: k4.10.0-37-generic
Network: Card-1: Marvell 88E8055 PCI-E Gigabit Ethernet Controller driver: sky2
IF: enp2s0 state: up speed: 100 Mbps duplex: full mac: 00:1d:ba:86:ca:c7
Card-2: Intel WiFi Link 5100 driver: iwlwifi
IF: wlp5s0 state: down mac: 00:21:5d:b5:9a:72

Any ideas? Thanks in advance
H. Stoellinger