Thank you page not displaying
I am fairly confident that your issue is that the WordPress redirect_canonical function is rewriting your unescaped CiviCRM URLs and redirecting to properly escaped URLs during the registration process. This would cause your session to be cleared during the redirect.

I recently fixed the as much of the code as I could find where CiviCRM generates URLs and updated it to produce valid URLs that wouldn't be caught by redirect_canonical. The latest CiviCRM has the fixes (so upgrading is as always recommended) but here are the relevant PRs in case you want to back-port them to your version:


https://github.com/civicrm/civicrm-core/pull/13043
https://github.com/civicrm/civicrm-core/pull/12969
https://github.com/civicrm/civicrm-core/pull/12968


As the question is also Paypal-related, you should also be aware of:


https://civicrm.stackexchange.com/a/27958/520


I haven't verified that jgillmanjr's fix works, but the logic looks right to me.