Undo function stops working

Hello!

my system: OSX 10.11.6
illustrator version: CC2017

Since the last CC update, for unknown reasons, the Undo function stops working ever so often. I cannot undo by using the shortcut nor by clicking it in the menu bar. This is accompanied by a minor side effect - when I try to save, the document saves properly - although the star indicating unsaved changes (* - to the right of the document title in the tab bar) doesn't disappear (which it should, after saving). I did not observe any other problems.

After restarting illustrator, things go to normal until it starts again. I do ignore it sometimes, and it fixes itself after some 5-10 minutes, until it breaks down again. Until now I wasn't able to replicate the error on purpose.

if anybody did encounter, or even better, by any chance solved this problem, please let me know.

thanks!

Andreas