Logged In customers couldn't view the product page but they can view the category page.
No issue with the Non-logged in or guest customers, they can view both the product page and category page.
Simply, try to disable all the installed third-party Magento extensions and flush cache.
Maybe it happens due to third-party extensions installation.
Otherwise, default Magento works perfectly.
Also you should activate your Magento's logs to be sure no internal errors are happening.