Poor dimensional accuracy, is the prusa mini is actually a bad printer?  

I forgot to mention before.. even it has probably nothing to do with your problem (but one never knows), I recommend you update your Mini firmware -the one you're using is kinda old.

Re

The box is the same with 0.5mm walls, although this one was without a middle wall. The result has been equally bad for both. absolutely I usually put them as close as possible, now I uploaded the last one I printed where I separated them on x, but all the boxes I print have become bad regardless of placement on the beds. even when placement was closer between them. I print other things flawlessly all the time, but I will definitely take a look at hotend and consider upgrading to bondtech. Yes I want to remember that first I tested PETG prusament and most recently I have run on PETG from addnorth. Many thanks for the tips!

There's a reason why we ask for 3mf file (and not for stl). It's because 3mf file contains all slicer settings (stl does not, of course) and from there we might see where/if user made a mistake. The thing is, it happens user says "all default settings, except temperature 240C", but then we realize that is not exactly true ("sorry, forgot to mention that").
In that sense, when you slice new print object, save project (3mf) and start printing... if it turns out you're not happy with print result, you can check with 3mf why's that.

Wish you lucky hand with hotend 🙂 

Problem solved

Thanks for all the tips, before I would take the hotend apart, I increased the walls to 0.8mm and now the box is perfectly printed 🙂

Yeah, with FDM printing considering wall thickness vis-a-vis extruder width is an important design aspect. At larger numbers it's not as critical but for thin walls it's key.