This is a shot in the dark (no pun intended), but there was a thread a while ago with someone having a similar problem. I'm sorry, but can't remember what thread it is. No matter what he did, he was having spray all over the place. Turns out that he had a crack in his filter basket. I'm almost certain that this isn't the same situation, but may be worth a thorough inspection of the basket.

I cannot see any cracks in the basket, but I ordered a new one anyway.

I found a home/biz espresso repair shop near my house that gets really good reviews. I will take the ECM to them this week, and I'll report their findings.

The repair guy said he did not want to work on the ECM because it was still under warranty. He said to set the pressure to 9 bars (with blind portafilter) and set the grind so the shot pulls at 8 bars. Which I did, and have not had a blowout since.

So at this point, I have no idea what was going on with the blowouts. Because previous to when they started, I'd never have a blowout even if the shot was pulling at 11 bars.

I switch back and forth between ridged and -less baskets all the time w/o issue.

Definitely not basket related.

Really sounds like you've got coffee that needs a very fine grind and a grinder that has problems with it. Or you're not grindng fine enough as you think when it starts clumping that you shouldn't grind finder. You should just grind finer until you have enough resistance and the only thing keeping you from grinding finer is burr rub on the grinder. And a new grinder with smaller burrs (Like the Vario and K3) will give inconsistent results the first couple of KG's as the burrs are breaking in. And smaller burrs do give more clumpng so that isn't any sign that you shouldn't grind finer. Maybe the burrs on the Baratza where just at their end of life. And clumping can easily be undone by doing WDT.

Havng the ECM at 9 bar instead of 11 will give better tasting coffee and less chance of problems in puck prep. That's always a good thing to do. As your machine is quite new I don't expect it to be a machine or basket issue.

And beans change every year when comng from the same origin so if you've been buying the "same" coffee for over 10 years then it's been totally different coffee over the years.

Thanks for the added info erik82.

"*9 acupuncture needles in a 3D printed handle available in a number of Etsy shops"

I used acupuncture needles and a wine cork. I have a drill press, and just drilled some fine pilot holes in a pattern matching the commercial tools. Works like a charm. And...looks super next to my elegant espresso machine...revealing my true cultural roots!