You don't say what version of Archicad you are using.
It is ideal to put this in a signature (go to the "Profile" link at the top of the forum) so that this shows on all your posts (together with you machine specs and operating system).
The later versions of Archicad will allow you to angle the side of slabs.
So you can still create your object but splay one side by 45°.
Then just mirror this object for the other side of the corner.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11