@momavujisic I need a bit more information to figure out what might be happening.
I’m unable to duplicate that error.
The error itself (if you’re on the latest version of M Chart) is happening at the point that M Chart checks a nonce value that WP should have generated to make sure the posts save/update submission is valid. Pretty standard stuff so it’s honestly quite weird that it’s failing at that point specifically.
Can you provide me with the version of WP you’re using as well as the version of M Chart and maybe even the data and chart type you’re trying to generate. I’ve got a fair number of users actively using the plugin and this is the first report of this issue so I’d like to chase it down if it’s repeatable.
Likewise with deleting charts. I’m not seeing any issues there either.
Subscribing to this in case you follow up with more info.
Marking as resolved since there has been no follow up with the OP.
