Parse failure with confusing debugging output

[GregoryTravis]

input.txt

grammar.txt I am getting a parse failure on some test input and I don't understand why. The debugging output contains this:

↙binopapp↙exp↙definition↙decls↙let↙exp↙definition↙decls↙start
<1:23>b + b }

This succeeds a little bit later: ≡binopapp↙exp↙definition↙decls↙let↙exp↙definition↙decls↙start
<1:28> }

But then the same parse fails after that:

≢binopapp↙exp↙definition↙decls↙let↙exp↙definition↙decls↙start
<1:23>b + b }

Grammar and input file are attached.