I couldn’t easily divide that module (issue with mutable objects in mixed mode). What helped somewhat was running backward() in profiler context, exporting chrome trace and looking at last successful ops, but that’s not a good or reliable solution. Luckily, I identified unbind from the above message, and indeed unbind was the problem.
Now, for some reason I failed to reproduce the failure in a small script, however I think it is somehow related to “Backward through view of unbind output 9” issue and seems to only fail in “legacy” jit mode (which is 1.6 default), so perhaps this will be handled in next release.