Condensate drain line freezing. Need an internal solution
If the drain line is accessible you could plumb it with a valve so that it goes into a tank during freezing weather. When not freezing it could go outside. What flow rate is coming out? If this is not possible or desirable, figure out how to direct the drain line into the drain under the lavatory.

Alternatively, you could put electric heating tape on the the part of the line that freezes.