Thank you very much to Pavel for the curl issue. I had to re-install curl and libcurl from LEDE repo and not Onion repo to get 7.54.1 (from Onion I got 7.52.1), but that,s not solved the issue another package was missing "ca-bundle".
After opkg install curl libcurl ca-bundle all is fine for curl.
Regarding to the current issue, thanks to ccs_hello. I guess you're right with the curret inrush explanation, but the issue is present only when Omega2+ is on battery, when alimentation is from micro-usb all seems to be ok. The same when i reboot the Omega2+ on battery with the USB stick already plugged. Then it's not a "show stopper" issue because this stick that made gateway between the "z-wave network" will be always plugged in. Just weird to see the different reactions of Omega2+
Finally, thanks again to css_hello, I understand that the USB-serial driver is "built-in"Omega2+ firmware because the device ttyACM0 is created when stick is plugged in, but the upper-half of the driver (the z-wave part) is supposed to be complied against Domoticz. Perhaps a incompatibility with packages compiled against Domoticz and some others compiled against Omega2+. May be I had to try to recomplie the Openzwave driver on Omega2+.