The lender says it is working on fixing the problem and has advised brokers to hold off submitting cases for the next couple of days.
The system is currently only processing completions and critical cases.
A spokeswoman for the lender says the fault is technical and not due to large business volumes.
In November the lender had to withdraw its tracker and fixed rate buy-to-let 75% LTV deals through the broker channel in order to manage service levels.
It also temporarily increased its service-level agreement for initial case assessments in order to manage expectations.
Its timeline to make an initial case assessment has temporarily increased to fourteen working days.