Hi,
You're saying that you have HikaShop 3 but your subscription to HikaShop Business ended more than 6 months ago and custom fields of the type ajax file are only available with the Business edition.
So do you really have the latest version ?
Did you download a HikaShop 3 package from somewhere else than our website ?
We actually fixed some bugs related to the custom fields of the type ajax file since the release of the 3.0.0 and added the patches on our end. If you download again the install package on our website and install it on yours, it should help with the problem.