None of what you describe is "normal", and it seems unlikely that it's anything wrong with your PayPal settings.

It seems like either a problem with your server or a problem with PayPal. It's hard to tell, as you only reported errors when you had PayPal URLs. In the case of your store URL, you didn't state what specific error showed on the page.

Also, in the cases when you got those errors, what shows in your Paypal debug logs?
And, importantly, what shows in your webserver's errorlogs as the cause of "page cannot be displayed", if any?
Have you tried in browsers other than IE to see if more information is shown?
Are there any PHP errors in the PHP debug logs? (ie: step 2a: https://www.zen-cart.com/tutorials/index.php?article=82 )