1.6.28 self-upgrade to 1.7.0-rc.17 not working

[NicoHood]

$ bin/gpm self-upgrade

GPM Releases Configuration: Testing

Grav v1.7.0-rc.17 is now available [release date: Thu Oct  8 01:43:55 2020].
You are currently using v1.6.28.
Would you like to read the changelog before proceeding? [y|N]  
Would you like to upgrade now? [y|N] y

Preparing to upgrade to v1.7.0-rc.17..
  |- Downloading upgrade [6.29M]...   100%
  |- Installing upgrade...    ok                             
  '- Success!  

PHP Fatal error:  Uncaught Error: Call to undefined method Grav\Common\Page\Pages::register() in /home/user/code/website.de_grav17/system/src/Grav/Common/Processors/InitializeProcessor.php:334
Stack trace:
#0 /home/user/code/website.de_grav17/system/src/Grav/Common/Processors/InitializeProcessor.php(147): Grav\Common\Processors\InitializeProcessor->initializePages()
#1 /home/user/code/website.de_grav17/system/src/Grav/Common/Processors/InitializeProcessor.php(54): Grav\Common\Processors\InitializeProcessor->processCli()
#2 /home/user/code/website.de_grav17/system/src/Grav/Console/ConsoleCommand.php(66): Grav\Common\Processors\InitializeProcessor::initializeCli()
#3 /home/user/code/website.de_grav17/system/src/Grav/Console/ConsoleCommand.php(107): Grav\Console\ConsoleCommand->initializeGrav()
#4 /home/user/code/website.de_grav17/system/src/Grav/Console/Cli/ClearCacheCommand.php(37): Grav\Console\Co in /home/user/code/website.de_grav17/system/src/Grav/Common/Processors/InitializeProcessor.php on line 334

I want to also note, that the upgrade guide has not a single note on how to perform the upgrade. It is only written in the blog post, but the UI is expected to be used, not GPM. This should be added to the docs after that issue got fixed.