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Getting this error after upgrading from 7.9 to 8.02
Exception: Laminas\HttpHandlerRunner\Exception\EmitterException: Output has been emitted previously; cannot emit response in /public_html/billing/vendor/laminas/laminas-httphandlerrunner/src/Exception/EmitterException.php:24 Stack trace: #0 /public_html/billing/vendor/laminas/laminas-httphandlerrunner/src/Emitter/SapiEmitterTrait.php(40): Laminas\HttpHandlerRunner\Exception\EmitterException::forOutputSent() #1 public_html/billing/vendor/laminas/laminas-httphandlerrunner/src/Emitter/SapiEmitter.php(27): Laminas\HttpHandlerRunner\Emitter\SapiEmitter->assertNoPreviousOutput() #2 /public_html/billing/index.php(0): Laminas\HttpHandlerRunner\Emitter\SapiEmitter->emit(Object(WHMCS\ClientArea)) #3 {main}
When I switch back to a really old template like the six or portal, the error does go away, but not any recent versions. Not really sure what is happening.
On PHP 7.3, but tried 7.2, 7.1, 7.0... to see if I could get the error to disappear.