Problem was related to some javascript for the select box. There was a function for a single select box but not a multiple select box - this fixed it:
if(theForm.elements[i].type == "select-multiple"){
var selectBox = theForm.elements[i];
var sbname = selectBox.name;
cpvalue = String(eval('record.data.' + sbname));
var NotifyArray = cpvalue.split(',');
for (var j=0; j < selectBox.length; j++) {
selectBox[j].selected = false;
}
for (var j=0; j < selectBox.length; j++) {
sbvalue = selectBox[j].value;
for (var k=0; k < NotifyArray.length; k++){
if (sbvalue == NotifyArray[k]){
selectBox[j].selected = true;
}
}
}
}
