Normally when we get the error back "Your merge fields are invalid" it means that you have attempted to send a value that does not fit in the type of merge field that Mailchimp is expecting. In this case, the one that sticks out is constituent id. We often set this up as a number in Mailchimp as most constituent ids are numbers. However as soon as you get a constituent id with a hyphen or some other non-numeric character in it, this breaks.
If you go to the audience and then select audience settings from the dropdown, then select the audience fields and merge tags as shown:
From there you can see the list of the fields for that audience. It will give you a type too. In many cases it will be text but if constituent id is number then this may be the problem.
Other similar errors may include:
Your merge fields were invalid.: FNAME - Please enter a value,LNAME - Please enter a value