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
If you are using Dreamhost like me you can create a phprc file inside /home/< (siteuser = the user your site runs under, and X.Y = php version eg 7.2)
Inside the phprc I added max_input_vars = 2000
You must then kill all the PHP processes to take effect using killall -9 php72.cgiif you were on PHP version 7.2
More info here https://help.dreamhost.com/hc/en-us/articles/214200668-How-do-I-create-a-phprc-file-via-SSH-