So far we cannot connect the Orange Pi Plus board to a DVI monitor using the HDMI port because it doesn't work. Is it a hardware problem, and in this case it will never work, or is it a software problem that can be solved in the future?

Same issue here, with a A20 SoC. HDMI/DVI adapter doesn't work.
This problem affects also BPi, so it's an old problem...

The adapter doesn't work because the DVI monitors not always support HDCP. It is necessary or use an adapter V1.4 HDMI to HDMI Converter Extractor Splitter SPDIF R/L+Video Audio out,HDCP Remover, US & EU Plug Adapter., at cost equal OPi2 or to disconnect HDCP in system.fex, but I didn't find mentions of HDCP there.

What abou Hdmi_hal_hdcp_enable=0 option ?

magicse,
How do you apply this? "What abou Hdmi_hal_hdcp_enable=0 option ?"