Hello Peter. Thank you for your reply.
max_input_vars did resolve the issue.
Strange though. This is a basic blueprint site on ZipWP. max_input_vars was already on 1000, I changed to 2000 and the issue resolved. What is strange is that there are only 18 fields, so MB must be sending a lot of hidden fields etc. Or ZipWP, is not really set to 1000 max_input_vars as the UI claims?
Appreciated.