some advice from others
A very extreme solution, not recommended unless you know what you are doing:

Take the bellows off, spray the inside with water to dampen all the cardboard, then work the bellows in and out. Once the card is all nice and damp then clamp it very firmly with pressure applied to the bellows frame, and to the interior of the folds so they are clamped tightly. Then leave to dry for several days at room temperature.

I have used this method successfully on new concertina bellows, but I've not tried it on big box bellows.

Disclaimer: I take no responsibility for the consequences if this goes wrong
2.It may also not be the cardboard but the surface cloth or paper. There can be an enenermous increase in bellows stiffness after the surface cloth is glued compared to what it was without it. If the cloth is glued carelessly, or the material is too thick, or you use wrong type of glue or too much of it, you get very stiff bellows. With same disclaimers as Theo the water trick might work applied on the outside as well...
3.I was revisiting this thread which started a year ago. I just found a new Ariette for very cheap, the owner bought it 2 years ago and never played it... The bellows is the older model with the brown tape, very stiff indeed! It stayed closed for 2 years.

I found that stretching it (lots of cracking noises!) often and to its maximum, and playing the thing as much as I could significantly loosened it, to the point that it is now very playable and not so stiff.

I understand it's a cheap chinese box but it sorts of grows on you, I spent time fixing little details, adjusting the stop mechanism so stops now move freely, etc. Not bad after al
4
There IS a simple and good tried method, used many times with 100% success.
Take the accordion, open the air valve or depress all the buttons, and stretch it to the limit, then some more.
You'll hear slight crackling from the bellows - it's the excess of glue, it's OK. Keep it there for a few seconds (10-20 if you can keep it up), then relax. Try playing - already wa-ay better, isn't it? Do it a few times It took 3 days of such excersize (once a day!!!) for my Parrott to loosen. It served me for 3/5 years, then I sold it. When I pick up Chinese made Ariette of the cheaper version (the better version has very good bellows indeed) - this strertching gets it to the point of playability. Otherwise it's just some funky box with semi-flexible middle part.
Hanging it with air valve taped over and weights attached also works, but marginally. You never know how much is enough. When you stretch it, you feel the pressure and do a better job.
I guess the key here is the quality of the cardboard. In cheaper boxes it's ... cheaper and softer and can be stretched this way.
On more expencive boxes it may not work this way. My Castagnari Lilly didn't stretch much. It was new and with very stiff bellows
Gargoyle piss off.