hmm the error log only lists "warnings" and no errors. I dont have access to an PHP 8.1.0 install nor does it seem that there is a xampp build there yet to debug. But the warnings hints about that this call here: https://github.com/joomla/joomla-cms/blob/3.10-dev/administrator/components/com_content/models/article.php#L308 fails can you check what happen when you change that code to the following and whether there are other issues reported when you set the error reporting to maximum?

		$item = parent::getItem($pk)
		if ($item)
		{

With log reporting set to maximum the page (fe/be) doesn't show up.
Below the output (with the ultimate error at the end):
Deprecated: The Serializable interface is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\input\src\Input.php on line 41 Deprecated: Return type of Joomla\Input\Input::count() should either be compatible with Countable::count(): int, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\input\src\Input.php on line 170 Deprecated: The Serializable interface is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in C:\Inetpub\wwwroot\www\libraries\src\Input\Input.php on line 31 Deprecated: Return type of Joomla\Registry\Registry::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 126 Deprecated: Return type of Joomla\Registry\Registry::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 439 Deprecated: Return type of Joomla\Registry\Registry::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 453 Deprecated: Return type of Joomla\Registry\Registry::offsetSet($offset, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 468 Deprecated: Return type of Joomla\Registry\Registry::offsetUnset($offset) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 482 Deprecated: Return type of Joomla\Registry\Registry::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 288 Deprecated: Return type of Joomla\Registry\Registry::count() should either be compatible with Countable::count(): int, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\registry\src\Registry.php on line 112 Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in C:\Inetpub\wwwroot\www\libraries\src\Application\WebApplication.php on line 1256 Deprecated: Automatic conversion of false to array is deprecated in C:\Inetpub\wwwroot\www\libraries\vendor\joomla\uri\src\UriHelper.php on line 50 Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in C:\Inetpub\wwwroot\www\libraries\src\Application\WebApplication.php on line 1305 Deprecated: Return type of Joomla\CMS\Session\Session::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in C:\Inetpub\wwwroot\www\libraries\src\Session\Session.php on line 298 Warning: session_name(): Session name cannot be changed after headers have already been sent in C:\Inetpub\wwwroot\www\libraries\joomla\session\handler\native.php on line 128 Deprecated: The Serializable interface is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in C:\Inetpub\wwwroot\www\libraries\src\Input\Cookie.php on line 105 Warning: session_cache_limiter(): Session cache limiter cannot be changed after headers have already been sent in C:\Inetpub\wwwroot\www\libraries\joomla\session\handler\native.php on line 235 Error: Failed to start application: Failed to start the session because headers have already been sent by "C:\Inetpub\wwwroot\www\libraries\vendor\joomla\input\src\Input.php" at line 41.
Adjusting article.php as mentioned does not help. With PHP 8.0 page is loading.

Actually PHP 8.1 introduces a lot of breaking changes and there will be a massive work to update the code of Joomla 3 and Joomla 4 in next months in order to have them compatible with PHP 8.1

#34952

Joomla 3 is now in maintenance mode and will only receive security fixes, so this is something (if valid) won't be fixed. Therefore, I'm closing this issue (Joomla 4 is working well with PHP 8.1 and does not have this error).