Weird slicer issue - Adding artefacts

Hi
I have file, bought from a reseller. It prints. The file needs a hole in it, which i have added in blender. See image of the resulting wire frame:

I now pull this into the slicer and now the slicer does something unextected:


Where does the odd horizontal spacing come from?

But there is more, sliced without support flat:


.

Everything looks great at layer 1 then it starts to bridge and fille the hole partially. If i add support it add support af if it needs to print in the hole that i created.

I dont really understand this. Can any one help?

Pulled it through Fusion360 repair tool. Now it at least slices. Waiting 8 hours for print :slight_smile: