We have not changed our API in quite some time. This error happens when the "tasks" parameter that you post to our api is empty or is invalid json. We did recently updated the version of PHP that we use, and this may have resulted in our json parser being more strict with regard to malformed json. I will look into this. Maybe you can also investigate on your end.
I think we may have found the problem and fixed it. There was a small configuration issue on one of our 6 web servers, which would explain why it was hard to reproduce. The problem was with the treatment of the backslash character inside your nested json array inside the meta field.
Please let us know if you continue to have this error.