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In case you aren't aware, there are clones inside groups inside clones inside groups, and etc.. I suggest hiding all the other layers, so that only the problematic one is visible. Then select all, then click the Unlink Clones button (on the command bar, looks like this
That
should unlink them all at once.
Hhm...but unfortunately, it doesn't....and I'm not sure why. It could be a lot of work finding all the clones and unlinking them. I'm not sure if there is a way to do it all at once now. Maybe Moini has an idea?
And plus, uncloning the first step also unmasks the first group. I'm not sure if that should be happening. I wonder if it could be a bug?
The shortest route to success might be isolating that layer, select all, and then Edit menu > Make a Bitmap Copy. This will create a PNG, which should export without any problem. Just be sure to hide the original multiple grouped/cloned/clipped/masked object.