TypeError: strlen(): Argument #1 ($string) must be of type string, array given

I am seeing an error with big pipe, call stack below.

Error shows Drupal core 10.4.5 and 10.4.6. PHP 8.2. both on Linux (Ubuntu) and Windows systems.
Error appears after an upgrade from a much older 10.1.3 version to 10.4.5. Also, a PHP upgrade from 8.1 to 8.2.
(On Drupal 10.1.3 and PHP 8.1 there were no errors and it worked fine).

Symptoms: A minimal custom block that is normally shown in the sidebar is no longer shown. At that time, consistently, the error below appears in the log.

I checked the code, the block shows a minimal form that builds four links and one submit button. I don't see anything special in it. The block is NOT cached with getCacheMaxAge() { return 0; }
Strangely, the block appeared fine during upgrade testing with existing content. Once we put it on live and generated new content/pages that show the block, everything broke and it disappeared everywhere, even on the older content pages.

Workaround: Turn off "Big Pipe" module and everything works fine again. So while the error appears in Unicode.php, I believe it is some sort of argument that would be passed down from Big Pipe.

I am not sure how you could reproduce it, but maybe the short stacktrace below gives a hint where a wrong argument may be passed.

Here is the stack trace:

TypeError: strlen(): Argument #1 ($string) must be of type string, array given in strlen() (line 395 of C:\...\\core\lib\Drupal\Component\Utility\Unicode.php)

#0 C:\...\core\lib\Drupal\Component\Utility\Unicode.php(395): strlen(Array)
#1 C:\...\core\lib\Drupal\Component\Utility\Xss.php(65): Drupal\Component\Utility\Unicode::validateUtf8(Array)
#2 C:\...\core\lib\Drupal\Component\Utility\Xss.php(123): Drupal\Component\Utility\Xss::filter(Array, Array)
#3 C:\...\core\lib\Drupal\Core\Ajax\MessageCommand.php(127): Drupal\Component\Utility\Xss::filterAdmin(Array)
#4 C:\...\core\lib\Drupal\Core\Ajax\AjaxResponse.php(43): Drupal\Core\Ajax\MessageCommand->render()
#5 C:\...\core\modules\big_pipe\src\Render\BigPipe.php(567): Drupal\Core\Ajax\AjaxResponse->addCommand(Object(Drupal\Core\Ajax\MessageCommand))
#6 C:\...\core\modules\big_pipe\src\Render\BigPipe.php(283): Drupal\big_pipe\Render\BigPipe->sendPlaceholders(Array, Array, Object(Drupal\Core\Asset\AttachedAssets))
#7 C:\...\core\modules\big_pipe\src\Render\BigPipeResponse.php(113): Drupal\big_pipe\Render\BigPipe->sendContent(Object(Drupal\big_pipe\Render\BigPipeResponse))
#8 C:\...\vendor\symfony\http-foundation\Response.php(423): Drupal\big_pipe\Render\BigPipeResponse->sendContent()
#9 C:\...\index.php(20): Symfony\Component\HttpFoundation\Response->send()
#10 {main}