Are the MS 6 battery powered or by USB? If they powered by battery I would change the batteries first.
Otherwise I would suggest to contact Aeotec, because it’s probably not a range/connection problem.
Warning: While pairing, a signal cannot ‘hop’ across the network. Also, for security measures, some devices pair in a special low-powered mode. This makes the range of that Z-Wave device temporarily small, to prevent malicious actions.
Always pair Z-Wave devices close to Homey, then place them in the right place. Within 30 minutes, the new network routes will be automatically established, and the device will become active.
Yes, -1 could be possible, depending on how the UVi (UV index) is calculated, as it is just a calculation of an analog value (that starts at 1 volts or something like that).
and -32768 as value means “invalid value”, still not very explanatory, but that’s all there is to it.
It could be just a badly iniated sensor, or bad firmware, but hard to say.
You could of course ask the developer of the devices and app (Aeotec) themselves, they probably can say more.