Manual:$wgAttemptFailureEpoch

Details
Certain operations are avoided if there were too many recent failures, for example, thumbnail generation. Bump this value to invalidate all memory of failed operations and thus allow further attempts to resume. This is useful when a cause for the failures has been found and fixed.

In practice this value is concatenated with the failure lookup key, and changing it will cause old failures to no longer be found.

Only thumbnail generation is currently using this setting.