Escape [ without html special chars

It's very unusual to have a value in the brackets of a checkbox (I've never seen it done). Are you sure you don't mean one of these?

<input type="checkbox" name="group[[+name]]-[[+id]][]" value="1">

<input type="checkbox" name="group[[+name]]-[[+id]]" value="1">

Try:

<input type="checkbox" name="group[ [[+name]]-[[+id]] ]" value="1">

Or

<input type="checkbox" name="group['[[+name]]-[[+id]]']" value="1">

["group"]=>
  array(1) {
    [" 2 "]=>
    string(1) "1"
  }

["group"]=>
  array(1) {
    ["'2'"]=>
    string(1) "1"
  }

It doesn't surprise me that this wouldn't work, MODX will try to figure out what kind of MODX tag [[[ is and of course fail, since there is no such tag. I think that two brackets without a following tag identifier is taken to mean "snippet", so what you have there is a snippet with a very strange name which of course doesn't exist so nothing is output. The inner full placeholder will be processed, so you'll end up with something like "group-1]"