Electronic – CAN bus polarity

It's not possible to find your bug remotely. The schematic is correct.

However you already found out that one of the connectors is not working. One or both CAN signal lines seem to be broken. Are you sure the soldering is really OK? And are the contacts clean and in a good shape?

You should also measure the resistance of one of the signal lines between certain points, e.g. between somewhere on the PCB and the contact inside the connector. Also, your cable to connect the next node may be broken.

A perfect, theoretical, differential system can run with no ground, because the input can always do (A-B) without worrying about the absolute values of A or B.

Unfortunately, CAN is not such a system, and the receiver will have limits to its 'common mode' capability, beyond which it be damaged. So you have to have a ground to keep the common mode voltages within limits. 'Common mode' just refers to stuff (voltage, in this case) which is common to both wires.

There are other ways of building receivers which are far freer of this sort of constraint - for example an opto isolator can do the (A-B) detection with maybe 1000s of volts of common mode offset. Transformers are often used too (e.g. in twisted-pair Ethernet, which has no ground). But for normal CANbus, you need a ground.