Hi and welcome here 
It is sadely a very common issue, answered many times on this forum
Please use the search function, the result is not guaranted but it’s worth to have a try:
https://forums.synfig.org/search?q=bone%20error%20sifz
And as I always recommend: Don’t use autosave and do regularly manual backups
Did you copy and paste any skeleton/bone?
Yes I did iam not going do it again