18. The OPT and COAX indicator is flashing at the same time. Why does this issue happen?

Are you using connecting to the 4.4mm port when the issue happens?

Please check whether the definition of the adapter/cable of your headphone is same to the K7 BT's 4.4mm port? Is your headphone the balanced headphone or the unbalanced headphone instead? 4.4 mm port:

The definition of most unbalanced 3.5mm port:

The balance output port has four output pins: L+, L-, R+ and R-. ("L" = "left channel" "R" = "right channel" "+" = "positive" "-" = "negative") However, the 3.5mm standard output port has only three pins: L, R and GND. Therefore, the 3.5 mm unbalanced output plug can not be converted into 2.5/4.4mm balanced output plug actually. Connecting the unbalanced plug to balanced jack may damage your device as in this case, the positive and negative terminals will contact and cause short circuit. If your adapter is for the 3.5 mm unbalanced output plug to 2.5/4.4mm balanced output plug, it is not suitable for the 4.4mm balanced port of the K7 BT. When using that adapter, the overload protection in K7 BT will be activated when the volume is in higher level in order to prevent the K7 BT for damaging.