Aluminum is at the upper end of what this machine can do. So you really need to have things dialed in for it to work well. Since I didn’t see it happen it is hard to say what went wrong. Here are some things to try.
If you are using this primarily for milling thin flat material you should make this much shorter. You should also try and get your tool further into your spindle. In the video you sent that material should have been able to be cut in 1 pass very quickly or 2 at the most. The closer you get the cutting end of the bit to the gantry the faster you should be able to cut because of increased rigidity. If you are using this for other things as well as cutting like I am, you should move the material to be cut much closer to the gantry. In that video I made I have a stack of wood under the aluminum. I actually made another and fit 1 more piece of wood under it and was able to cut it faster. Think of it this way, every time you cut the distance from the material to the gantry in half you should be able to cut 2 times faster.
You can move the spindle up in the z direction Ideally you want the bottom of the collet just lower than the bottom of the z tubes to increase rigidity.
You should cut down the z length. When your tool is hitting the work surface your z motor mount should be almost hitting the gantry. If you have more than this you are swinging the mass of the z stepper around unnecessarily.
The 1.5mm bit is very unforgiving even though it is removing less material you might want to get it working well with a 3mm bit first and then figure out how to slow it down for a 1.5mm bit.
You might want to try a lower depth of cut, that puts less torque on the machine. There is a limit though material does have to be removed or you will just work harden it and never cut it.
You can try a coolant as well if your feed rate or spindle speed is off you can actually work harden the aluminum and heat up the bit very fast. to cut without coolant you need to have your feeds and speeds dialed in.
did the spindle slow down when it hit the aluminum? 400W might not be enough power. I believe the dewalt I am using now is 600W, but in my first aluminum videos I was using a 400W flex shaft rotary, and it did fine.