VMU does not work connected to the controller

[gelcila]

I have around 4 VMUs that don't work when connected to the Dreamcast controller. In fact, the control "disappears" and is no longer recognized by the Dreamcast until I remove the VMU.
These VMUs work perfectly with batteries outside of the Dreamcast and there are still saved games there.
Its boards are in perfect condition and I have always stored the VMUs without batteries to avoid the risk of leakage.

I did simple procedures like cleaning the contacts and resetting them, but nothing helped.
I believe that some internal component, like some type of fuse, has blown and needs to be replaced, but there is no such information on the internet.
There's no way I'm one of the few people with this problem on VMU.
I looked for help all over the internet and couldn't find an answer, even though I found many reports with the same problem.
Does anyone know how to fix this?

Before anyone asks, I have others perfectly working VMUs on my controllers, on all Dreamcast ports at the same time, so it can't be a problem with the controller ports or the Dreamcast controllers themselves.