This is an FYI post because upgrading your version of IE to 11.0.110 will fix the issue. Adding it here so the answer is available for anyone receiving complaints! I did not report it as a bug because it's not Adobe's bug and is fixed in a newer version of IE.
Using RH 2019 (not classic). This does not happen in outputs from similar projects in RH 2017. I did not try classic 2019.
Have tried in both the Azure_Blue and Charcoal_Gray outputs and the same thing happens, it's not layout-specific as far as I can tell.
Table of Contents (TOC) must have books that are not linked to topics but do contain topics.
Topic must have breadcrumbs somewhere in the topic.
Only happens in Internet Explorer 11.0.105 and before. Fixed in IE 11.0.110.
Description of error:
Open output in Internet Explorer.
Use the TOC to browse to a topic that is first*** in the list of topics below a book with no linked topic.
In that topic, click the link to the folder that your topic is in. This will result in one of two errors:
If the project is housed on web site or intranet, "403 - Forbidden: Access is denied. You do not have permission to view this directory or page using the credentials that you supplied."
If opening from Windows File Explorer you see the directory tree for the folder.
***Note that the error doesn't occur if you click the second (or lower) link in a folder but instead produces the desired behavior, which is to open the first link in the folder.