Internal error: Unable to save render elements. Please report us this problem at https://corona-renderer.com/forum
This message is triggered when some internal Corona error occurs during rendering. Aside from regular Internal Error message, this one also informs you that Corona has failed to save render elements. You can still manually save render elements from CoronaVFB.
See: How to save all render elements at once? , What elements are there?
To make sure this bug is fixed in future, please report error at https://corona-renderer.com/forum/, and try to provide as much data as possible, ideally including scene where error is reproducible.
You can still manually save render elements from Corona VFB - see above links.