Many newer ATX PSUs don't provide -5V anymore, since it's no longer required by the latest ATX spec. The hardware monitor reading is most likely wrong, but to make sure, better verify it with a multimeter (if your PSU still has a -5Vrail (white cable), if it doesn't have one, you can safely ignore the error).
The -5V rail is rarely used on mainboards and modern hardware, but might be needed by old ISA cards.

5u3 wrote:

Many newer ATX PSUs don't provide -5V anymore, since it's no longer required by the latest ATX spec. The hardware monitor reading is most likely wrong, but to make sure, better verify it with a multimeter (if your PSU still has a -5Vrail (white cable), if it doesn't have one, you can safely ignore the error).
The -5V rail is rarely used on mainboards and modern hardware, but might be needed by old ISA cards.

I also hope that PSU is providing -12V, as that signal is still used by the PCI bus and some peripherals. Only the ISA bus has a -5V signal, and not all implementations (Tandy 1000s) of it implement it. On the plus side, very few cards are known to use it. If the CUBX-E does not have ISA slots (I know my CUBX without the -E had one) you won't have a problem. However, but 5u3 and I own a very important ISA card that does use the -5V signal.

many thanks for the info! yes, CUBX-E has two ISA slots. i'll try using a little bit older PSU in a few days.

important ISA card that uses -5v ? is it the LAPC-I ? 😁

Quite some ISA VGA cards use -5V for the analog signal generation. Some sound cards use it too. Very early mainboards need the -5V for their DRAM chips.

The original reason for the -5v being on the ISA bus was for IBM's diskette drive controller, which used it. Other manufacturers used different controllers, many of which did not use it.