Warning: simplexml_load_string()

I found a very old issue like this one, but although it still appears active, nothing seems to have been done on it, and it applied to a dev version of Collapse Text. I've tried setting up my page as described in the Readme file, but I get a long string of error messages (see attached txt document). My instructions to collapse the block of text are ignored as is the title. All that displays is the default title above the text to be collapsed. It does work in that clicking on the title will collapse the text.

My code looks like this:

[collapsed title="Why not a Welcoming Church or Denomination?"]
<p>My content here.</p>
[/collapse]

I've tried using [collapse collapsed title="title"] and several other variations, but nothing seems to work.

When I look at the generated source code (using Firebug), it looks like this:

<fieldset class="collapse-text-fieldset collapsible collapse-text-default-title form-wrapper collapse-processed"><legend><span class="fieldset-legend"><a href="#" class="fieldset-title"><span class="fieldset-legend-prefix element-invisible">Hide</span> Use the arrow to expand or collapse this section</a><span class="summary"/></span></legend><div class="fieldset-wrapper"><div class="collapse-text-text">
<p> My content here.</p> 
</div></div></fieldset>

It looks like to me that the module/filter is parsing the markup partially correctly, but I can't find what's generating the errors and what's happening to my title. By the way, I also tried defining the title in <h2></h2> tags above the [collapse collapsed][/collapse] markup. That didn't change anything, except that the title displayed with the "Use the arrow ..." hyperlink below it; I still got all the error messages.

Any help that you can give me on getting this to work would be greatly appreciated. I'm sure I must be doing something stupidly wrong, but I can't figure out what it is. If I could get it to work, this would be a fantastic module and something that I really need.