While running a program compiled in the Arduino IDE 1.8.5 and then using the .ELF file inside LabCenter Proteus 8.0, for an Arduino Nano ATMEL Mega 328P, for a code involving the use of the SoftwareSerial.h Library, hence involving several loops, without user interaction, I am getting the following error:

Invalid opcode 0x0024 at PC=0x0096

Now I am completely lost. According the AVR Instruction Set Manual such opcode should (not sure about this?) be RETI - Return From Interrupt at the Program Line (?) no.150 = 0x096. The full code has 779 lines plus several library calls, and the line 150 happens to fall just before the loop() function (?), which should never repeat, being just a declaration.

So, how should I debug this error? Evidently I am not asking to solve the problem for me but, how should I proceed for debugging it and find the real error?

I have been unable to find in Proteus a tool for "Debug Stop if Error"

I have been unable to isolate the error in a simple way to share. It should be related someway with interruptions used by the library SoftwareSerial.h, because at some modifications of the program, the code freezes at an interruption return. I am working on that, but advancing really slowly actually.