Hi,
1. We don't know of any problem with the registration system of HikaShop 2.3.4
2. Before updating an extension, especially if it's an important one for your business, it's always wise to try it on a copy of your website, just in case. That way, if something happens after the update, you can have someone look at it while still have your website selling as usual.
3. The Fatal error message is not related to HikaShop. This error happens because your website's PHP memory limit is too low. Just ask your hosting company to increase it (or do it yourself if you have access to the memory_limit parameter of your php.ini and it will be ok.