In newer PHP-Versions the count of input-fileds per formula (POST) will be limited to 1000 (unverified information). It seams that this limit is already installed in certain
It's always good to just stringify some of the data you're submitting – rather than change this limit within PHP. For example, if you're sending in stuff via Ajax, or via a form field, just stringify the field that's causing the problem.
The limit is on the 'number' of fields and not the 'length', if that makes sense.