Espresso machine drain line keeps clogging

I have an espresso machine plumbed in for both water intake and drainage. The water intake is working fine, but the drain line that came with the machine keeps clogging. The symptom I'm observing is that the drain tray (that should be connected to the drain line) keeps overflowing from around where the drain line is connected (instead of through it) and creating puddles on the counter.

I checked the line and confirmed that:

1. There is a flat or slight downwards gradient at all points.
2. It's securely connected at all points.

I also never drain any thick coffee grounds, though small sediments probably get in there periodically when I'm clearing the portafilter or brewhead.

Why does this keep clogging? Is there something I can do differently to prevent future clogs?

Here are some pics. The drain line is the white / transparent flexible line. It goes from under the coffee machine, down through the countertop and into a cabinet, then sideways into the under-sink cabinet. In that last one, I make it do a slight loop to keep a downward gradient, then it connects to the sink's trap.