'System is temporarily unavailable' error for View Email as Web Page

The 'View Email as Web Page' (VAWP) link stays active for as long as the URL Expiration policy allows, however it can also be inactive due to several reasons discussed in this article.

The typical reason for an unavailable VAWP link and displaying the error message is the following: 

• The link will be broken if the data in the Data Extension/List, used at the time of the send, is overwritten, updated or deleted. This is related to how the VAWP link works.  The 'View as webpage' link is different to the data in the original email sent for a particular subscriber and the data contained within an email is static, whereas the data contained within the 'View as webpage' link is pulled each time the page is rendered. 
• Deleting or overwriting the Data Extension will affect the view as webpage when a subscriber is trying to view the web links. This occurs as the view as webpage link works by collecting source data of subscriber and performs lookups to create the email as a webpage view.
• Contact deletion will affect the view as webpage when a subscriber is trying to view the web links.

Workaround

1. Implement Send Logging 

To use this, you'll create a new Data Extension for this purpose and use the 'SendLog' template and reference this at the time of send. 

Send Logging

Once you have implemented a send log, this will then require additional AMPScript in your emails, which logs the send and required personalization data to a central data extension. When the VAWP link is clicked, the page will pull the required data from this SendLog Data Extension, as opposed to the original SF Data Extension. See the documentation Display Send-Time Content Contextually.
 

2. Import Activity 

An alternate option would be to use an Import Activity within the Marketing Cloud interface to import the report into a Data Extension and send via a Guided Send. This way you can set the retention policy as per your requirements.