I started having the same problem yesterday, although preceded by occasional "dead input" issues over the previous ~10 months that would quickly resolve by unplugging the XIM Apex and plugging it back into the PS5. That workaround stopped working pretty abruptly yesterday (the input devices would go dead faster and faster and now cannot be used past maybe 1 second)
As of now, it's completely unusable and none of the attached devices can provide input (in my case I have a Sony Nav, a mouse, and a Nacon controller connected to the XIM Apex through the "mouse adapter"). If I plug in Sony's PS5 controllers directly into the PS5, that works so I think the PS5 itself is not the problem. I've swapped in some different input devices and cables just in case, but no change in behavior.
The XIM Apex glows blue slowly as normal. There is the small white LED on the input mouse adapter that stays lit (FWIW). I am running the latest beta firmware (20211012). I was running the beta firmware from January when the problem happened. I tried flashing the latest beta firmware just in case, but it made no difference.
I looked to buy a new XIM Apex + mouse adapter to test out, but it seems that none are available.
Any options or possible other workarounds?