Hi,
Hard to say. It could be a bug in HikaShop, it could be a bug in another extension, or another extension conflicting with HikaShop.
Or it could even be an issue with not enough resources for some specific action.
The best is to get the real PHP error message behind that generic Apache error message.
If it doesn't display on the page, it usually means that it's because of how the web server is configured, and in that case, you need to check the PHP error log of the server directly for the fatal error message corresponding to the issue.
If you don't know where to look for the PHP error log, you might want to ask your hosting support as it depends on how your server is configured.