Hi,
That was indeed a memory limit issue. Note that 128M for the memory limit is not "massive". It's actually the default for PHP 7.
However, using 64M for a normal page processing is indeed a lot. Normally a normal Joomla page is between 10 and 40M. If you look at the error message, you can see that the error happens in the image helper which tries to allocate 22M of memory at once. This indicates that it's trying to generate a thumbnail of a big image and the image resolution is so big that it takes one third of all the memory needed to process the page. So unfortunately, that's not something we can avoid in HikaShop.
The solution is indeed to either increase the memory limit like you did or use smaller images for the products with big images.