Q: Open4-RLink (= Open4-Base + OP4-RLink-ADP) cannot read or verify Flash through SWD. Other operations (Erase, Prog, Debug, etc.) work fine. How can I correct this?

A: Unsolder the R9 resistor from the OP4-RLink and solder a 20K resistor instead (or higher or even nothing at all, it should be fine in most cases).

The first production batch of OP4-RLink-ADP extension board version 1.1 had an incorrect R9 resistor: 4K7 instead of 20K (or more).

Note that if you have problems during operations other than Read (Dump) and Verify Flash, then probably this is NOT the explanation (and solution) for your problem.