I got it to work. I took the path with the tree, and, in Inkscape, did a Path->Break Apart. This revealed about 20 different little nodes that were not connected to the tree path. I deleted those nodes, and now it seems to work.
It’s too bad the message just says “An error occurred”. Something like “Could not calculate path” would be so much more informative.
But I’m glad it’s working again. My GF has been a workhorse since I’ve gotten it. I’ve had a couple of hickups. I’m sure I’ve now successfully printed over a 1000 times, with only 3-4 prints that gave me trouble. So overall, I can’t complain.
Thanks everyone for helping me debug it.