POST request failed with UpdateException

Nextcloud version (21.0.1)
Operating system and version (???)
Apache or nginx version (???):
PHP version (7.4)

I am trying to update nextcloud. I tried using the built-in updater and at the first step “Check for expected files” it says “Parsing response failed” and “500 Internal Server Error”.
I am grateful for any ideas on what to do. I guess its good to know, that I am completely new to SSH, just taking my very first steps.
I tried looking for already existing answers but couldn´t really find something.

Thank you in advance

Is this the first time you’ve seen this error? : No

Steps to replicate it:

  1. Start built-in updater

Output of nextcloud-upater.log

[info] checkForExpectedFilesAndFolders()
[error] POST request failed with UpdateException
[error] Exception: UpdateException
#0 /var/www/virtual/###/html/updater/index.php(1361): Updater->checkForExpectedFilesAndFolders()
#1 {main}
    [0] => 0

The output of your Nextcloud log in Admin > Logging: (all these errors are not occurring in the exact moment of the update but are older)

Error	core	Error: Class OCP\Constant not found 

Error	PHP	Doctrine\DBAL\Exception\DeadlockException: An exception occurred while executing a query: SQLSTATE[40001]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction at /var/www/virtual/riw/html/3rdparty/doctrine/dbal/src/Driver/API/MySQL/ExceptionConverter.php#37

Error	index	Exception: Argument 1 passed to OC\Core\Controller\LoginController::tryLogin() must be of the type string, null given, called in /var/www/virtual/riw/html/lib/private/AppFramework/Http/Dispatcher.php on line 218