While building out a transition plan from Drupal to Wordpress, after getting CiviCRM setup on Wordpress, I tried making a test contribution. However, all attempts resulted in `Pending (Incomplete Transaction)` despite showing as successful in the sandbox.

As part of troubleshooting, I tried making a test contribution on the production site (which is the one currently running Drupal) since I had previously made successful test contributions. However, this too lead to the contributions being marked as `Pending (Incomplete Transaction)`.

One thing to note is that "Real" contributions have still been coming in fine on the production site.

I'm aware that there was an issue previously per [this](https://civicrm.stackexchange.com/questions/22049/fatal-error-using-paypal-in-version-4-7-29) StackExchange post, however it appears that should have been fixed in `4.7.30`, whereas I'm running `5.8.2` (both on the production and dev sites for obvious reasons).

No errors seem to be showing relating to this in any of the `error_log` files on the production or dev site.

Any further troubleshooting advice would be greatly appreciated.

Additional Information
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I'm using Paypal Standard.