Unknown objects

Martin,

Hold Ctrl/Cmd and press E.

This toggles between GPU and CPU, the former sometimes being a bit unruly.

This is what GPU is about:
https://helpx.adobe.com/illustrator/kb/gpu-performance-preview-improvements.html

GPU is faster, sometimes too fast, so it gets ahead of itself.

Graphics cards are included in the system requirements:
https://helpx.adobe.com/illustrator/system-requirements.html
https://helpx.adobe.com/illustrator/system-requirements/earlier-versions.html
https://helpx.adobe.com/illustrator/system-requirements-old-versions.html

Apart from an inadequate card, the issue can be caused by lack of driver updating, lack of proper recognition of the card by the computer, and whatnot (including rifts in the space time, moon, weather, and the fact that Illy (job description Adobe Illustrator) sometimes moves in mysterious ways).

Here are a few more pages about GPU troubleshooting:
https://helpx.adobe.com/illustrator/kb/gpu-performance-errors-troubleshooting-workarounds.html
https://helpx.adobe.com/illustrator/kb/illustrator-gpu-performance-driver-update.html