i can't understand how these gaskets would stop the flange sliding towards the heating element when i turn the screws in.

Your photo seems to show that the rim of the flange isn't aligning to the threads. This could mean you aren't engaging the threads that are there because you are cross-threaded, so that you are not properly engaging the threads between the flange and the boiler base.

You write that you have an '85 Europiccola, which is this.

This larger thread pitch flange works for machines that are after 1983. Before then Europiccolas had a flange with a finer thread pitch and heating elements that screw onto the boiler instead of being bolted down, unless you find an aftermarket fine thread pitch flange that allows a bolted replacement heating element.

You have to install the base of the machine first, it will take up room along that big threaded tube.
Install the boiler body into the base, THEN mount the heating element plate.

Edit: sorry, I didn't watch your video, I just looked at the picture without playing it. After watching your video and listening to it, its clear that either your boiler threads or the ring threads are partially stripped. The proper way to fix it would be to buy a new ring or new boiler, or both; but that is very expensive. If it does thread on partially like your video suggests, then you might try building the threads up slightly; but that's not an approved way of fixing it. Perhaps some very thin metallic tape wrapped over the threads? I've never tried that, though. Depending on the thickness of the metallic tape, it may not be possible to screw the ring back on... so it looks like at the very least you should try getting a new ring and seeing if it continues to slip with the new ring. It is cheaper than getting a new boiler

But, before spending money, try what I suggested above. Get 2 new gaskets, one above the base and 1 below it, and put the boiler into place with the ring... see if it continues to slip when you do that. The video sort of shows threads that are still in pretty good condition, so you might get lucky and have the bad threads only lower down like where it is in your video.

THANK YOU!

you've been the first to understand what i'm talking about!
will try to reinforce (somehow) the threads on the boiler and secure it, i am thinking maybe adding some Loctite thread-locker to it and then install the flange, hoping it will keep it from moving...

You need to be very careful with Loctite. It is cyanoacrylate based, which I think mostly has a working temp that is close to the boiler operating temp. It will probably fail and since this is a boiler with pressurized hot water inside, this is very dangerous... you are asking for trouble. Check whatever sealant you use, it will have a working temperature range clearly published on the container and on their website. It MUST have a working temp that is a safe amount higher than your boiler working temp. If its even close, don't chance it. You must leave a safety margin for error (error... as in, forgetting to put enough water into the boiler and overheating it by mistake) That's why I suggested you try metallic tape instead; it does not have a temperature limit.

The video suggests it is your RING that may be slipping, although I cannot really tell unless I can see the threads themselves... but a new ring is around $65? something like that. I'd try that first, or else try small strips of metallic tape... if you use the tape, don't run a whole strip all around the threads, you'll never get the ring back on. Run one strip, maybe 1" wide or so, along one side and a matching 1" strip on the opposite side (180 degrees around the boiler on opposite side). If that's too tight then make the strips 1/2" and try again... etc.

I have had this issue come up a couple times and have tried various tightening methods and using Loctite. I finally got fed up and fixed the issue by replacing the standard fibrous gaskets with a gasket making compound. Something along the lines of this https://www.permatex.com/products/gaske ... ne-gasket/ is what I used. This stuff is meant for high temp applications with significant compressive force applied, so it can absolutely handle this application. It also has a tacky rubber kind of texture to it which I believes helps hold everything in place. I have had my machine setup with this stuff for over a year now with no degradation in the gasket material and zero slippage, with fairly minimal tightening required on the flange.