We can validate a Node created from JSON against the schema with the Node.check() function¹⁾. This function will throw an exception if the Node is invalid. However, we currently don't get much information from the thrown exception²⁾.

Currently, the best approach seems to be to add some console output to the only place where we do have access to the information about which Fragment is causing the problem. It is in prosemirror-model/dist/content.js, in the function matchFragment. Starting at line 288 this could look like the following:

ContentMatch.prototype.matchFragment = function (fragment, from, to) {
    var this$1 = this;
    if ( from === void 0 ) from = 0;
    if ( to === void 0 ) to = fragment.childCount;
 
  if (from == to) { return this }
  var fragPos = from, end = this.expr.elements.length
  for (var ref = this, index = ref.index, count = ref.count; index < end; index++, count = 0) {
    var elt = this$1.expr.elements[index], max = this$1.resolveValue(elt.max)
 
    while (count < max && fragPos < to) {
      if (elt.matches(fragment.child(fragPos), this$1.attrs, this$1.expr)) {
        count++
        if (++fragPos == to) { return this$1.move(index, count) }
      } else {
        console.error('Fragment not allowed in ' + this$1.expr.nodeType.name);
        console.error(fragment.child(fragPos));
        break
      }
    }
    if (count < this$1.resolveValue(elt.min)) { return null }
  }
  return false
};