Without being absolutely certain, it might be that the role that you're part of has insufficient data rights on that contact you're trying to save.
I would check the role and see if the data rights are set to "Update".
But it might also be, like you're saying, just this one contact - and that there are some corrupt data on the given contact which returns the error when you try to save.
Do you know if it happens on other contacts as well? Or only this one? I see that you're saying that it's only this one, but there might be more, unless you've tried them all.
It's quite difficult to help without being able to test this myself. I would advice maybe contacting support.