Hi,
I think that your server have a execution time limit.
It means that a PHP script can't run more than 30 seconds (the usual time limit).
Because we use a PHP script to send the file to the user (in order to secure the download and let the file accessible like that), it means that the script have to send the file under 30 seconds.
We use a command to ask the server to change his limitation (to have no time restriction) but some hosting refuse it (it can be too CPU consuming for them to let users use scripts without time limitation).
Please contact your web hosting company to have more details about that and to have a good way to propose a secure download of a file.
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.