Thanks for the test account. When DAVx⁵ queries the resources, the server sends:
<?xml version="1.0" encoding="utf-8"?>
<D:multistatus xmlns:a="urn:ietf:params:xml:ns:carddav" xmlns:D="DAV:"><D:response><D:href>/SOGo/dav/c%26m@example</D:href><D:propstat><D:status>HTTP/1.1 200 OK</D:status><D:prop><a:addressbook-home-set><D:href xmlns:D="DAV:">/SOGo/dav/c&m@example/Contacts/</D:href></a:addressbook-home-set><D:group-membership/></D:prop></D:propstat></D:response></D:multistatus>
which causes this DAVx⁵ error:
Couldn't parse urn:ietf:params:xml:ns:carddavaddressbook-home-set EXCEPTION org.xmlpull.v1.XmlPullParserException: unterminated entity ref (position:TEXT @2:247 in okhttp3.ResponseBody$BomAwareReader@abfda3c)
As you can see, the & is not escaped in the href element, which is invalid XML and can’t be parsed. So, this seems to be a server problem and it seems to be related to the bug report you have posted.
If there are any specific indications that this could be a DAVx⁵ problem, please let us know.