Offset Tracking Error: NullPointerException

Hi all,
I am performing offset tracking on Sentinel-1 amplitude images: L1 Detected High-Res Dual-Pol (GRD-HD). I have performed the process on a total of 16 images, and am now running into this error once I get images dated 2018-05-13 to present:
Error: [NodeId: Offset-Tracking]
I have already updated to the latest version of SNAP.
Any suggestions as to why I am getting this error and how it can be fixed?
Thank you!