Had the same issue.
Andreas suggestion seems to work! I'm able to save profile changes.

Im having the same issue. Running 7.2 and the latest Joomla right now. All of a sudden saving profile as a user was resulting in an Error 0. After deleting the line suggested here, the page successfully "saves" and reloads, but none of the changes are saved.

User can't edit anything including description and avatar image. Even deletion of avatar doesnt work.
Would really appreciate some help as this is a core feature I'm hoping to use!

I can make changes from the back-end as an administrator, but not as a user on the front-end.

I can confirm that this is *somewhat* of a problem still...

I've installed the latest version as per Fotis' suggestion.
I no longer receive the Error 0, however the changes will only save under some conditions..

It has to do with the password/password verify fields.

If you do not supply a password, or supply the password in both password+verify fields, the k2 profile as well as any other custom user plugins save.
If you only put the password in the password field (a lot of browsers pre-fill this for you) then only some of the data saves.
In this case:
* The basic user data saves.
* The user.profile plugin data saves.
* The k2 plugin data does not save.
* My own custom user plugin (same code as user profile, just extra fields) does not save.

Thanks for coming back to this Fotis...

I found the problem after a day or two..
It seems my joomla template was missing the messages definition:

<jdoc:include type="message" />

When you only put one password in (even it if is correct) then Joomla thinks you are trying to change the password and fails because the passwords don't match.

I did not know this was the reason because I did not see the error message.

The strange thing is that without the messages definition, the save does not work, but when you include the message definition the save *does* work fine..

Either way, my conclusions are:

1. It's a joomla core issue.
2. It's not an issue when you properly build the template...