Can you take a screenshot?
Does this happen with different pattern files, or has it happened only on one pattern file?
I know reproducibility is mandatory to fix software. Up to know I don't see steps to reproduce. If I discover something I will try to run within gdb.
At this point, you must upload your .val and .vit files. Usually there's a problem in the pattern in these circumstances. That doesn't mean you've done something wrong, it means we need to investigate.
Please click the Edit button for this issue in the upper right of the screen, and attach your .val and .vit files. You may need to attach one at a time.