[Solved] ESP8266 Exception(3) error when using crc16.Modbus in an if condition

The problem is unlikely to be an if. Do some experiments - e.g. assign a constant to crcCalculate and in the next instruction the result crc16.Modbus. See how the code behaves when you perform an operation on crcCalculate e.g. crcCalculate++

I used to have a strange situation - a series of functions that returned "String" (in uppercase). In a couple of projects this works without problems, but in one there were strange reboots about 2 times a day. I thought it was interference or power problems. However, I had to change the code and after the corrections the processor rebooted almost continuously. I started to trace the problem and it came out that the String returned outside the function was no longer good. A strange issue as this works in many places. I rewrote all the code so that String is passed into the function (as a pointer) and the problems stopped. So far I don't know what was wrong - a compiler error or something in ESP.
In summary - there is not necessarily a bug in your code.

len-2 ... - are you by any chance inserting a negative string length?

FreelancerPL wrote:

I have an incomprehensible error in the ESP8266.

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I suggest you first install yourself the Exception Stack Trace Decoder for ESP8266. Life will become easier .
What IDE are you using?