Hi,
Yes, we got the access working.
The issue is indeed what Nicolas was talked about in his previous message with the max_input_vars variable set to a maximum of 1000 input boxes per form.
You need to see with your system administrator or hosting company to increase that PHP variable if you want to keep adding more cities to your custom field.
Otherwise, the fields are limited and some inputs are not received by PHP ; which generate the error you got.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.