I found an old thread that may or may not be helpful.
OPC-DA to RSLinx Classic (Emulated PLC) - General Discussion - Inductive Automation Forum
The piece that stuck out to me was in step 1 set the default authentication level to default. On my computer that setting is set to connect. I’m very unfamiliar with OPC-DA, but I am wondering if having that setting set to connect allows the connection but not read/write operations.