But apparently it was because I did the initialization of this string inside the main routine.
Bizarre that it is throwing a warning for the string 'stringTemp' and not for the long variable 'index'.
Both are only used inside the main routine.

This is the complete code which is now running without a warning :

//assigning inputs to variables, these variables are READ-ONLY
double input(0) inputGLAS; 
double input(1) inputGFT; 
double input(2) inputPMD;
double input(3) inputRA;
double input(4) inputTEXT;
double input(5) inputP_K;



//assigning variables to outputs, these variables are WRITE-ONLY
string output(0) stringOUT; //value to send to y0 output
char stringTemp[80];



//the init procedure is executed once when the REXLANG function block initializes
long init(void)
{
	
	return 0;
}

//the main procedure is executed repeatedly (once in each sampling period)
long main(void)
{
	
	
	long index = 0;
	
	//char stringTemp[80];
        stringTemp="";

	strcat(stringTemp,"Vergeet+niet+volgend+vuilnis+buiten+te+zetten+:+");
	
	if (inputGLAS!=0)
	{
		strcat(stringTemp,"Glas");
		index=index+1;
	}
	
	if (inputGFT!=0)
	{
		if (index!=0)
		{
			strcat(stringTemp,",+");
		}
		strcat(stringTemp,"GFT");
		index=index+1;
	}
	
	if (inputPMD!=0)
	{
		if (index!=0)
		{
			strcat(stringTemp,",+");
		}
		strcat(stringTemp,"PMD");
		index=index+1;
	}
	
	if (inputRA!=0)
	{
		if (index!=0)
		{
			strcat(stringTemp,",+");
		}
		strcat(stringTemp,"Restafval");
		index=index+1;
	}
	
	if (inputTEXT!=0)
	{
		if (index!=0)
		{
			strcat(stringTemp,",+");
		}
		strcat(stringTemp,"Textiel");
		index=index+1;
	}
	
	if (inputP_K!=0)
	{
		if (index!=0)
		{
			strcat(stringTemp,",+");
		}
		strcat(stringTemp,"Papier+en+Karton");
	}
	

	stringOUT=stringTemp;
	return 0;
}

//the exit procedure is executed once when the task is correctly terminated
// (system shutdown, downloading new control algorithm, etc.)
long exit(void)
{
	/* PUT YOUR CODE HERE */
	return 0;
}

char stringTemp[80];
stringTemp = "";

if you want to be sure that the array is empty. The warning will disappear.

Cheers,
Jan