osC's standard PayPal module does not include this line, which appears to be essential to the handshake working right between osC and PayPal.
tep_draw_hidden_field('rm', 2) .
See the attachment for installation details. It may be applicable also to PayPalIPN, but I've not gone there yet.
I'm guessing PayPal introduced this after 2.2 was released. It's covered in PayPal's installation guide, but what a needle-in-haystack for me to find in the Forum!
Credit for the find belongs to forum user TFO, whose profile is blank, so I can't acknowledge him/her by name.
My first contrib, so be sure to do your backup!