Right, it sounds like the BMS is doing its job and it's tripped. When it cuts off the output, you will still see voltage if measured with a voltmeter, but it will drop to zero with a load.
You need to check all the individual cell voltages. Most likely there is one that is out of range. I do this by inserting little wires into the BMS connector and attaching them to my voltmeter.
"One test is worth a thousand opinions"