Dripping is normal when the AC is running. Will drip a lot more on very humid days. And if you have a problem like the indoor evap coil icing up it can drip as it melts off with no equipment running.
It did not drip when cold ac was ON before. Why is it dripping now when cold ac is ON?
It's no different than the glass of iced tea causing a ugly stain on the fine wood furniture because of condensation . Some days maybe , some days maybe not , some days all day and all night or maybe not .
The question is why was it not dripping when cold ac was on during summers before?
something is not right here.
where is main drain line?
Could the clog be cleared from the drain line shown in photo? Assuming that is the secondary/emergency drain line.
The unit is built in the attic on the second floor.
Yes it was, you just didn't notice.
If the cold is working, it will produce condensation.
You wrote that water was coming out of the pipe --- that's what its supposed to do.
It would be a problem if water was NOT coming out of the condensate drain.