Já tentei todas as etapas do tutorial https://helpx.adobe.com/br/illustrator/kb/fix-unknown-error-issues.html
E ainda assim não consigo salvar arquivos em .ai.
Já desinstalei e reinstalei o Illustrator... continua bugado.
Podem me ajudar?
Try to turn OFF Preferences > File Handling > Save in Background.
We are sorry for the trouble. The product team is aware of the issue, and they are currently working on a fix. I would request you to please be patient & stay tuned for updates. In the meantime, please use the workaround suggested by @Ton Frederiks. I hope it helps!