Preview mode not working in 13.7.2 - LazyLoad.min.js 404

chris-isnt · April 9, 2025, 2:04pm

Hi friendly Umbraco folks,
I have a website (originally v7) that I’ve upgraded recently from 13.4.1 to 13.7.2 and preview has stopped working - just a blank page, and the device title is {{previewDevice.title}}.
The console shows a 404 on umbraco/lib/LazyLoad.min.js. There’s no /lib folder under /umbraco on disk - even on a clean install where preview works - so can anyone tell me where these js files are being accessed from, and what might be stopping them being found?
Thanks!
Chris

seanrockster · August 27, 2025, 11:52am

Still not working in 13.9.2

jacob · August 28, 2025, 5:20am

Do you perhaps have a rewrite rule in place to enforce the use of trailing slashes? We have seen this issue before, and it seems there is a fix:

github.com/umbraco/Umbraco-CMS

Preview mode does not work if URL trailing slash is removed

opened 01:15AM - 18 Feb 25 UTC

closed 01:10PM - 18 Feb 25 UTC

dave0504

type/bug state/reproduced area/backend affected/v13

### Which Umbraco version are you using? (Please write the *exact* version, exam…ple: 10.1.0) 13.5.1 ### Bug summary After implementing a URl rewrite rule in IIS to remove trailing slash from page urls, I noticed that preview mode had stopped working. I reproduced the issue after removing the rewrite rule, and testing locally, removing the trailing slash form the url in browser. /umbraco/preview?id=someNodeId - results in a blank page. Viewing the network tab in Chrome shows that /lib/lazyload-js/LazyLoad.min.js returns 404 ![Image](https://github.com/user-attachments/assets/f89d0e48-2908-40e2-a04c-65bec9968c48) ### Specifics _No response_ ### Steps to reproduce Remove trailing slash from url in preview mode ### Expected result / actual result _No response_

You can likely implement a rewrite rule specifically for the Backoffice to make it work.