Thanks for credentials and sorry it took that long but as you know I was on sick leave.
I investigated the issue, initially assuming that there is something wrong in Collect (maybe after switching to OkHttp). However, now I'm pretty sure it's a problem on your (server) side.
Your server returns error code 500 Internal Server Error
I used an online tool to test http requests https://reqbin.com/ and:
GET with https://wepi.org/odk/formList and credentials you have shared with me returns a list of all available forms and it works in Collect as well as you have noticed.
GET with any
downloadUrl from the received list eg. https://wepi.org/project/ODKXML/token/5885e55a67b34
500 (Internal Server Error) (same like in Collect) and
<meta charset="utf-8" />
<link rel="stylesheet" href="/css/site.css" type="text/css" />
<h2>Web questionnaires<br />for epidemiologists and health professionals</h2>
<input type="button" id="link_vzn" name="link_vzn" value="Powered by" onClick="document.location.href='http://www2.voozanoo.net';" class="btVzn" />
<h3>An error has occurred</h3>
Unfortunately an error in Wepi has occurred. It has been logged and sent to our software team. If you get this message frequently, please contact us at email@example.com to tell us more about the circumstances that make this occur. We are sorry for any inconvenience
this has caused you. Try clearing the cache (Firefox: ctrl-shift-del > cache, Internet Explorer: Tools > Safety > Delete browsing history > Internet and temporary web files)in your web browser.
<a href="/index/about/token/5885e55a67b34">About WEPI</a> | <a href="/index/faq/token/5885e55a67b34">FAQ</a> | <a href="mailto:firstname.lastname@example.org">Contact WEPI team</a> | <a href="/index/legal/token/5885e55a67b34">Legal info</a>
<div id="footer_copyright">© <a href="http://www.epiconcept.fr">Epiconcept 1996-2013</a></div>
so to sum up, your server has some problems with GET requests. It works for form list (in Collect and using the tool I used) but it doesn't work if you want to download a particular form.
I'm not an expert in that area but maybe your server requires something more, some special header in GET request maybe if you want to download forms?
Please try to contact wepi email@example.com as described in the revived content.