See the description of the issue under OpenCart 2.3.x. I have confirmed it under 18.104.22.168 as well as 22.214.171.124. I have submitted a GitHub issue and pull request that fixes it for 3.0.x (see https://github.com/opencart/opencart/issues/5861).
The solution is posted ... right above. If yours is not working after applying this fix, then you are reporting a different issue with PayPal as your post needs to be elaborated.
The most generated errors being found on Opencart forum originates from contributed programming. The increased post counters are caused by redundancies of the same solutions that were already provided prior.
Programmer / Opencart Tester
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