This “E” sign will light ON in the battery details tab of the App. Check the flashing LEDs on the string of cells.
The BMS message starts at the Begin Board. When the next board received the message, the LED will flash and the board will send the message to the next board.
The position where the LED stops flashing is the location of the problem. It can be that board or the previous board. Check if the communication wire goes from the “OUT” in the previous board to that board’s “IN” connector.