In the F77 binding, the subroutine plstrf2c() called twice with an integer array instead of a character array as the second argument. This happens in the subroutines plstring() and plstring3(), in the source file bindings/f77/sfstubs.f:
call plstrf2c(string,s1,maxlen)
where s1 is an integer array rather than a character array.
Closed as the Fortan 77 bindings have now officially been removed from plplot.