askvortsov here are the errors I've seen with 8.1.1:
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: Return type of Flarum\Foundation\Config::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/customer/www/mydomain.com/public_html/forum/vendor/flarum/core/src/Foundation/Config.php on line 61
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: Return type of Flarum\Foundation\Config::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/customer/www/mydomain.com/public_html/forum/vendor/flarum/core/src/Foundation/Config.php on line 56
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: Return type of Flarum\Foundation\Config::offsetSet($offset, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/customer/www/mydomain.com/public_html/forum/vendor/flarum/core/src/Foundation/Config.php on line 66
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: Return type of Flarum\Foundation\Config::offsetUnset($offset) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/customer/www/mydomain.com/public_html/forum/vendor/flarum/core/src/Foundation/Config.php on line 71
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: Return type of Tobscure\JsonApi\Document::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/customer/www/mydomain.com/public_html/forum/vendor/tobscure/json-api/src/Document.php on line 222
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: mb_strwidth(): Passing null to parameter #1 ($string) of type string is deprecated in /home/customer/www/mydomain.com/public_html/forum/vendor/illuminate/support/Str.php on line 350
[20-Jan-2022 19:34:32 UTC] PHP Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /home/customer/www/mydomain.com/public_html/forum/vendor/illuminate/database/Query/Builder.php on line 836
[20-Jan-2022 19:34:32 UTC] PHP Fatal error: Uncaught Laminas\HttpHandlerRunner\Exception\EmitterException: Unable to emit response; headers already sent in /home/customer/www/mydomain.com/public_html/forum/vendor/laminas/laminas-httphandlerrunner/src/Exception/EmitterException.php:19
Stack trace:
#0 /home/customer/www/mydomain.com/public_html/forum/vendor/laminas/laminas-httphandlerrunner/src/Emitter/SapiEmitterTrait.php(36): Laminas\HttpHandlerRunner\Exception\EmitterException::forHeadersSent()
#1 /home/customer/www/mydomain.com/public_html/forum/vendor/laminas/laminas-httphandlerrunner/src/Emitter/SapiEmitter.php(27): Laminas\HttpHandlerRunner\Emitter\SapiEmitter->assertNoPreviousOutput()
#2 /home/customer/www/mydomain.com/public_html/forum/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(98): Laminas\HttpHandlerRunner\Emitter\SapiEmitter->emit(Object(Laminas\Diactoros\Response\HtmlResponse))
#3 /home/customer/www/mydomain.com/public_html/forum/vendor/flarum/core/src/Http/Server.php(44): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#4 /home/customer/www/mydomain.com/public_html/forum/index.php(26): Flarum\Http\Server->listen()
#5 {main}
thrown in /home/customer/www/mydomain.com/public_html/forum/vendor/laminas/laminas-httphandlerrunner/src/Exception/EmitterException.php on line 19