Dear Chris,
I have just experienced the exact same problem. I get the exact same warning message when I try Treeanotator on my star-BEAST run (v. 1.7.5)
I have run more than 100 parallel runs of the same star-BEAST xml-file and I keep getting this message.
I have tried to discard 50% of the first trees (burnin), but still get this message.
The individual gene-trees can be combined in Treeannotator without this message appearing.
The resulting combined tree is wrong. The species-tree have negative branch lengths, I can 'straighten the branches' by keeping the height of the branches, but the the nodes in my basal branches of fish are more than 1000 mio. years old, and these old (incorrect nodes) are present in both gene-trees and the species-tree.
Where you able to solve this problem?
Thanks in advance for your time