Side-chaning

Anyone else having problems with side-chains in N11.1?
Whenever I instantiate one, I get what looks like a runaway feedback loop though one of the stem buses (out to a summing mixer). Then it settles after a while> Same thing happens with the next one, or when loading the project again.

eg side chain from the vox to a FabFilter Pro C2 or MB on another track. Same goes for stock Steinberg plugs. The instant the side chain is turned on (& the send appears in the vox track), off it goes.

Update: now resolved, thanks. User error.