Where the files are saved (Network Drive/ System HDD/ Shared Folder on System /External HDD)?
When did this start happening? Are there any recent changes made to your system? For example, any antivirus, system updates or web extensions installed recently.
We have recently released an update for Illustrator 2020 (v 24.2) which contains some stability fixes. If you are not already on v24.2, then we recommend you to update to latest version and let us know how it goes.