Unable to save the illustrator file in windows 10 pro

I started having the same issue after updating Illustrator this morning. I've found that there was a font conflict (I use Connect Fonts and saw one was automatically activated that has been a problem in the past). After changing the font to another one, I can save the file.

Perhaps try outlining fonts and save under a different file name or save a copy to test if there is a font issue. If it then saves you can, by process of elimination in changing one font at a time, find the problem font.   ??

This was only a workaround for me since the font is used on a template that the whole department uses, but it's a workaround nonetheless. Hopefully a fix shows up soon or I'll move back a version with Illustrator.

Please tell us the detailed steps that lead to this.

Is it just one particular file or all files?

Where do you want to save?