To narrow the error down:
please can everyone confirm, that this error is isolated for EDITING existing pages and - vice-versa - the VE works flawlessly while CREATING and SAVING new pages without errors. At least this could exclude, that Parsoid isn´t invoked correctly via LocalSettings.php .

In T270377#6971946, @Seb35 wrote:

wfLoadExtension( 'Parsoid', 'vendor/wikimedia/parsoid/extension.json' );

Hello Seb,

All those Wikis with not very tech-savyy users or webmasters, or sitting on servers without root access and/or are using a one-click-install instance of MW - in mind , I feel it appropriate that the priority of this issue has to be escalated, so those wikis are operational even for those (which will cover the majority of all wikis out there, run by private individuals).

Running on a server with all libs installed Chris was mentioning, we still run into the 500 errors. The weird thing is, this seems to be a native editor issue - not a misconfigured parsoid instance.