We have the fix available in the latest release. Please update Illustrator using Adobe Creative Cloud application to the latest build for version Illustrator CC 24.0.1

Please refer to our knowledge base FAQ – https://illustrator.uservoice.com/knowledgebase/articles/1844590 if you face difficulty in update.
Or get in touch with us at any of the other support channels – https://helpx.adobe.com/support.html

Thanks
Anish Kumar

This issue is fixed in 24.0.1 beta build and you can download it from the location: https://www.adobeprerelease.com/

You need to join the Adobe Illustrator CC pre-release program after logging into the above site under the “Available Programs” tab. Your Adobe Creative Cloud login credentials will work perfectly here.

After joining to the pre-release program click on the “My Programs” tab and click on “View Program” button under the heading Adobe Illustrator CC.
In the downloads section builds are posted under title “24.0.1 Build 1”.

Please do let us know if the issue still persists on your system with this build or you have any other queries regarding Illustrator.

NOTE:

Please note the build is from our pre-release program. The information discussed within the program is confidential. Your participation and your knowledge of this information cannot be shared outside of the people in pre-release group per your acceptance of the Pre-release Agreement.

Following is the workaround for 24.0.0

If you do not use custom Adobe PDF presets/joboptions or not heard of it then follow the below steps
1. Navigate to user presets folder
Win: appdata\Adobe\Adobe PDF\Settings
Mac: ~/Library/Application Support/Adobe/Adobe PDF/Settings
2. Rename the folder to “Settings old” and create a new empty folder “Settings” in the same place
3. Launch Illustrator and check if you can save the document

If you use custom Adobe PDF presets then follow below steps
1. Go to Edit > Adobe PDF Presets
2. Check PDF compatibility of PDF presets that are not default
3. Make a note of presets that use PDF compatibility version 1.3
4. Quit Illustrator
5. Navigate to user presets folder
Win: appdata\Adobe\Adobe PDF\Settings
Mac: ~/Library/Application Support/Adobe/Adobe PDF/Settings
6. Move the presets identified in step 3 out of this folder
7. Launch Illustrator and check if you can save the document

This issue is reproducible at our end and investigation underway.

This issue is caused due to one of the PDF preset using custom Flattener preset. Users who run into this issue can try the following and update us in the comment

1. Go to Edit > Adobe PDF Presets
2. Check PDF compatibility of PDF presets that are not default
3. Make a note of presets that use PDF compatibility version 1.3
4. Quit Illustrator
5. Navigate to user presets folder
Win: appdata\Adobe\Adobe PDF\Settings
Mac: ~/Library/Application Support/Adobe PDF/Settings
6. Move the presets identified in step 3 out of this folder
7. Launch Illustrator and check if you can save the document

Hi,
Apologies for the inconvenience caused to you due to this issue.
We are unable to reproduce this issue in house.
Can you help the Illustrator team to isolate this issue further so that we can nail this down?
If yes, then please let me know the following:
1)Kindly provide some video , Test file(Via File→Package) & some steps with which you are facing issue and share with us at ShareWithAI@adobe.com in a Zip file.
Please mention the title of this issue as the subject of the mail.

Thanks and Regards
Ankit Goyal
Illustrator Team