I can't tell from picture but is the lock ring fully rotated to the stops and seated?
did you lube the seal and make sure sender mounting plate was centered when locked down?
I would make sure it's the connection leaking and not a penetration in the plate and replace seal..
if the sender fittings are questionable, replace it.. cheap enough..
based on the quality of replace parts these days, wouldn't be surprised if either or both defective
I would not use any sealants or third party sealing aids but rather restore a reliable factory seal..
good luck!