Thank you Sonja. I was able to figure out these errors, but now I am stuck a a new one.
The first error was caused by a now outdated version of the Custom Add Fields plugin. The second error was caused by the breaking changes regarding hooks on the 3.4 update. Now I fixed that issue, by properly renaming the hook parameter.
But now I am stuck a at a new error message that relates to that hook and some custom methods I had set up in the hooks to handle writing to structure fields within structure fields within the pagebuilder plugin. As the pagebuilder plugin is obsolete now, I am really at loss how I can transfer the existing setup to the current version of Kirby.
Here is the error message I am getting (again after updating the page):

Here is the hook I am using:
if ($newPage->intendedTemplate() == 'project') {
$yamlbuilder = $newPage->eventcontextbuilder()->yaml();
foreach ($yamlbuilder as &$eventcontextblock) {
if($eventcontextblock['showevents'] == 'false') {
$eventcontextblock['parsedinfo'] = ' [Alle Termine ausgeblendet]';
} else {
$eventcontextblock['parsedinfo'] = '';
}
foreach ($eventcontextblock['eventbuilder'] as &$eventblock) {
$startdate = date('d.m.Y', strtotime($eventblock['date']));
$time = $eventblock['time'];
$enddate = false;
if($eventblock['enddate'] !== '') {
$enddate = date('d.m.Y', strtotime($eventblock['enddate']));
}
$parseddate = $startdate;
if($enddate) {
$parseddate .= ' - ' . $enddate;
} else if ($time) {
$parseddate .= ' ' . $time;
}
$eventblock['parseddate'] = $parseddate;
$statusdivider = $eventblock['eventtype'] ? ' ' : '';
if($eventblock['showevent'] == 'hide') {
$eventblock['showstatus'] = $statusdivider . '[ausgeblendet]';
} else if($eventblock['showevent'] == 'hideafter') {
$eventblock['showstatus'] = $statusdivider . '[eingeblendet bis zum Termin]';
} else {
$eventblock['showstatus'] = '';
}
}
}
$newPage->update([
'eventcontextbuilder' => Data::encode($yamlbuilder, "yaml")
]);
}
Here is a thread, where you @texnixe, helped me to come up with this solution, to give this some context.
I am not sure what is causing this error now and how to resolve this, as I am not quite certain what changed behind the scenes in regard to the replacement of the pagebuilder by Blocks.
Any idea how I could solve this?
Thanks!