I'm guessing it's a due so overflow, you should be able to just pull the standpipe in the overflow out to check the stand pipe for clogs
If there's no clogs in the stand pipe, remove your flex line completely and check that, if that doesn't solve it pull the pvc pipe that goes from the flux hose into the sump out and clean that
If you do all that you'll have you're system running normally in no time
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