I am using the %%view_email_url%% personalization string in all my emails to provide the subscriber with an alternate view that is platform-independent. These links are going dead, increasingly over the past few weeks, only days or minutes after the initial send occurred, even though the same URL was functioning a moment earlier. This was never a problem a few months ago. My "URL Expiration" setting, under Admin, has always been set to 730 days.
Instead of the email message, the subscriber sees a blank white page with the text: "The system is temporarily unavailable. We apologize for any inconvenience. Please try again later." But the situation is permanent, not temporary. Simultaneously another email's %%view_email_url%% link works successfully, proving that "the system" is not unavailable.
As my only hunch, could this possibly be caused by some change to the subscriber's record in the data extension that was used to target them? Or not? Our data extensions are updated or overwritten on an hourly basis -- although any given subscriber would never disappear; just their attributes could change.
The criticality of this functionality relates to its usage here in a customer service context, where we frequently need to see what the customer saw.