Sounds like the default New Document Profiles are corrupted.
I would follow Rartemass’s advice and either run a repair on you Illustrator installation, or do a complete reinstall of Illustrator.
Or
Illustrator Repair Kit. Adobe Illustrator repair file software for corrupted *.ai files. Illustrator Repair Kir assists to repair AI file data.
For more: https://www.illustrator.repair/