To verify whether this has happened, you first enable component debug mode in the AD job service. If the nsTraceLog then contains a message that looks like "2014-07-01 16:44:43 +02:00 - Info - Got schema object via configuration part." , then the fallback mechanism was used. If the schema was read directly without fallback mechanism, this would contain the wording "via server context" or "via forest context" depending on your configuration.
If the fallback mechanism was used, you need to determine why the account that was configured to be used for synchronisation with the domain cannot access the schema. There are various potential reasons for this, and once the problem is resolved, the next schema readout should work normally.