After upgrading to version 15.4 the Edit for Lists, Slides, Announcements etc all continually reload when entering Edit mode.

For example in Content->Lists->Select base list item->Select specific list member will cause a never-ending loop making it impossible to edit most content types.

 
Also log-webclient-error?sf_site=435fcaa-b87-7876-9878-7scshachb21 is failing with 401 Unauthorized

CustomErrors setting in web.config, the setting is OK and should work fine, but in a specific Sitefinity site it may cause the 401 problem.

<httpErrors errorMode="Custom">
<clear />
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" path="/404-error" responseMode="ExecuteURL" />
<error statusCode="403" path="/404-error" responseMode="ExecuteURL" />
<error statusCode="500" path="/404-error" responseMode="ExecuteURL" />
</httpErrors>

Delete the <httpErrors> section in web.config, it is causing a problem/conflict which cause is undetermined. In order not to loose the functionality of <httpErrors> replace them fully with Sitefinity error pages / httpErrors functionality.

To set it up:

Go to Administration -> Settings -> Advanced -> Pages -> Custom Error Pages and create error page rules for (see screenshot below).

 
Create entries in this functionality for all error codes covered by <httpErrors> rule.
Code 404 | 404-error
Code 403 | 404-error
Code 500 | 404-error

The page 404-error must be a page in Sitefinity, it cannot be HTML file.

Sitefinity documentation, custom error pages https://www.progress.com/documentation/sitefinity-cms/administration-custom-error-pages