Why do some houseplants suddenly start dripping sap? Help please!

That sticky stuff, is not sap. Its called "honeydew" and its secreted by a pest called scale.

You might try spraying with rubbing alcohol. Mix 1 part alcohol with 8 to 10 parts of water and add a squirt of liquid soap. Be sure to spray all leaf and stem surfaces thoroughly.

Repeat this treatment again in 5 to 7 days to catch any that you missed the first time. After that, check your plant weekly to see if they return.

If that don't work there are some organic mixes out there, be sure to get the one with a silicone base. Check at a dedicated garden shop.

I would also isolate the plant, otherwise it might spread to your other plants.

Thank you very much for this advice

Your Welcome!