RSS Graffiti explains it this way ...
You’re on this page because RSS Graffiti displayed the following message:
“Facebook returned an access token that expires in x minutes. Please see our troubleshooting guide for help in resolving this issue.”
This message is related to these errors which may be appearing in your activity log:
“RSS Graffiti does not have the proper credentials to publish to the target. No credentials found for target/page type”
“Facebook error code: 190: Error invalidating access token: Session has expired at unix time …”
These notices indicate RSS Graffiti doesn’t have a long-term access token for your profile. Access tokens allow RSS Graffiti to publish on your behalf when you’re not logged into Facebook. Whenever you visit the app, we attempt to request both a never-expiring access token for pages and a 60-day token for profiles and groups, so that publishing can continue for as long as possible without any action from you.
However, Facebook made some recent changes which caused problems granting long term access tokens and there are certain cases where we are only able to get a 60-minute access token, which is nearly useless after you’re logged out for more than an hour.
The app publishers at RSS Graffiti seem a little frustrated with the message, too.
If you continue to see the short-lived access token message, you may be subject to a confirmed Facebook bug related to this problem. You can subscribe to the bug here to try and raise Facebook’s awareness of the problem so that they quickly implement a fix.
One user, Justin Tazyn, sums up Facebook's response to the issue perfectly ...
"This has been high priority for over 2 months. Is it really high priority?"