Hi Rémi,
I recently experienced the same situation and engaged OSS after going through similar troubleshooting steps, so maybe the same solution will work for you.
With Solution Manager SP10, BPMon uses the TMW RFC destination (if defined) rather than the read RFC destination (which I missed when reviewing note 784752).
Can you check the authorizations on SMTM<SID> in the managed system? I suspect it doesn't have the necessary S_CTS_ADMI authorization object.
Hope that helps.
Tyson