[Al Skierkiewicz]

Max,
This is a typical failure for this type of connector. The screw is designed for solid wire or a stranded wire that fully fills the opening or is covered in a ferrule. When you tighten the screw on stranded wire, the individual strands "ride up" the screw threads until only a few are actually be contacted at the end of the screw. In this way you are trying to force the several hundred amps through essentially a #16 or smaller equivalent wire. The result is heat. Even if you get many strands tight initially, some of the strands will work out and that will loosen the bite on the remaining strands. Again, heat is generated. What appears to be rust may actually be the plating on the parts detaching under the heat stress. That coupled with any chemical reaction with the tape and adhesive will lead to the pitted, textured look of the connector. Screw terminals that actuate a clamp to hold the wire do not suffer this failure. The screw pushes against the back of the clamp body which in turn applies the force to the stranded wire.
Are there any markings on the connector? I will bet there is something like 8-2 or 10-4. That is the range of wire that can be used with this connector. Many electricians read that as #8 solid or #2 stranded. When we had to use this type of clamp on the old brass distribution blocks, we folded over the #6 so that it would completely fill the void before we tightened the screw.