Unknown text printing on Serial Monitor

Question

I want to use the Serial Monitor for some debugging, but it prints something else.

Edit- This only happens if I use the attachInterrupt() function

I tried another program that does not use interrupts and there it works.

My code:-

/*
YF‐ S201 Water Flow Sensor
Water Flow Sensor output processed to read in litres/hour
Adaptation Courtesy: www.hobbytronics.co.uk
*/
volatile int flow_frequency; // Measures flow sensor pulses
unsigned int l_hour; // Calculated litres/hour
unsigned char flowsensor = 13; // Sensor Input
unsigned long currentTime;
unsigned long cloopTime;
void flow () // Interrupt function
{
   flow_frequency++;
}
void setup()
{
   pinMode(flowsensor, INPUT_PULLUP);
   Serial.begin(9600);
   attachInterrupt(0, flow, RISING); // Setup Interrupt
   sei(); // Enable interrupts
   currentTime = millis();
   cloopTime = currentTime;
}
void loop ()
{
   currentTime = millis();
   // Every second, calculate and print litres/hour
   if(currentTime >= (cloopTime + 1000))
   {
      cloopTime = currentTime; // Updates cloopTime
      // Pulse frequency (Hz) = 7.5Q, Q is flow rate in L/min.
      l_hour = (flow_frequency * 60 / 7.5); // (Pulse frequency x 60 min) / 7.5Q = flowrate in L/hour
      flow_frequency = 0; // Reset Counter
      Serial.print(l_hour, DEC); // Print litres/hour
      Serial.println(" Lhour");
   }
}

Data from Serial Monitor:-

--------------- CUT HERE FOR EXCEPTION DECODER ---------------
⸮⸮⸮⸮⸮⸮ISR not in IRAM!
User exception (panic/abort/assert)
--------------- CUT HERE FOR EXCEPTION DECODER ---------------
Abort called
>>>stack>>>
ctx: cont
sp: 3ffffef0 end: 3fffffc0 offset: 0000
3ffffef0:  feefeffe feefeffe feefeffe 00000100

3fffff00:  000000fe 00000000 00000000 00000000

3fffff10:  00000000 00000000 00000000 00ff0000

3fffff20:  5ffffe00 5ffffe00 3ffef0a4 00000000

3fffff30:  00000001 00000000 3ffee34c 40201e72

3fffff40:  40100402 40202e51 ffffffff 40201e84

3fffff50:  feefeffe 00000001 3ffee34c 40202399

3fffff60:  00000000 00000001 3ffee324 4020129c

3fffff70:  00000000 feefeffe feefeffe 3ffee38c

3fffff80:  3fffdad0 00000000 3ffee34c 40202448

3fffff90:  3fffdad0 00000000 3ffee34c 40201079

3fffffa0:  feefeffe feefeffe feefeffe 40201a80

3fffffb0:  feefeffe feefeffe 3ffe84e4 40100cfd

<<<stack<<<

Why is this happening?

Hardware-ESP8266 based NodeMCU