Symptom
The preview in the new web client (commonly known as vnext) does not work and shows a message like "preview unavailable, click here to download the file" in version 22.12 (December 2022).
Clicking the download link fails to download the file, however.
The preview and download in the Classic web work fine.
The network request (you can see it in the browser developer tools) returns a "500" server error with a message like "session not found".
Details
This happens for three known reasons:
- URL case sensitivity - resolved by M-Files.
- Having the web client set up the old way - resolved by configuring web access anew.
- Proxy changing the URLs - resolved with the information here.
if you access the vault with an URL like this (any capital letter in the "vnext" portion of the URL):
but this may work fine
If you access the vault with a lowercase "vnext" it should be fine.
Otherwise, the network request for the preview ends with a 500 server error and a message like "session not found".
In some cases, if you have set up the web access prior to version 22.10 and have done so by manually creating a web application called "vnext" in your IIS, this can also cause such behavior.
Solution
This matter is being investigated for a solution built-in the product with ID 166042 that will be present in the release notes if/when resolved.
A fix in the product is available in the 23.1 release (January 2023).
Workaround
First, try to access the vault with lowercase letters for the "vnext" portion, like this:
If that does not suffice, double check how your web client is set up. Follow the current instructions starting from this article: Setting Up Web and Mobile Access to M-Files.
After following through the M-Files wizard, double check the web site in IIS - it must not have the "vnext" and "wopi" web applications. The wizard tries to delete them but if it does not have rights, you may have to do that manually.
Using the Classic Web client or the Desktop client is also a viable workaround.
Notes
There may be other reasons for previews not showing up, explore the following resources as well:
