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...fixed it ;)
Actually you was probably right before. C3 does not have USB OTG. It only has fixed function USB CDC Serial + JTAG. So no generic purpose USB that could do HID or mass storage or anything. So USB is there just for flashing and console so you don't need to have usb to serial chip on devboard. S3 has same fixed serial/jtag interface + USB OTG on same pins so you need to select at runtime one or another (you can't have both) but C3 only has that fixed function one.
Did it myself, just my mistake :) Doing in in a hurry.
...fixed it ;) Thx for the review :)