Diagnosis
The smell isn't from the filter—it's from mold and mildew growing on the cold, wet evaporator coils inside the unit. That's where all the moisture condenses, and if spores are present, they'll thrive there.
How to Fix It
You need to clean the coils. Unplug the unit and let it sit for a day so everything dries out. Remove the front grille and filter. Using a spray bottle, mist the coils (the finned radiator-looking part behind the filter) with a 50/50 mix of white vinegar and water. Let it sit for 15-20 minutes to kill the mold. Then, gently rinse the coils with a light spray of clean water from a spray bottle (don't soak the electronics). Let it air dry completely for 24 hours before plugging it back in.
If That Didn't Work
If the smell persists, the mold might be in the drain pan underneath the coils, which you can't see without disassembly. In that case, using a dehumidifier cleaner foam (like what's made for AC coils) can help reach those areas.
Heads Up
Never use bleach or harsh chemicals on the coils—they can corrode the aluminum fins. Vinewater is safe and effective. Running the unit on a higher fan speed in a drier space can help prevent the smell from coming back by keeping the coils drier between cycles.