Checking the whole error log again, I guess the important part is directly at the beginning:
Argument #1 ($legacyOption) must be of type array, null given in /mnt/web021/e3/85/51320785/htdocs/catzeyes/store/src/Core/Payment/PaymentOption.php:329
So, 'convertLegacyOption()' expects an array but receives "null"
Checking the PaymentOption.php at line 329 and following I can see these lines:
public static function convertLegacyOption(array $legacyOption)
{
if (!$legacyOption) {
return;
}
if (array_key_exists('cta_text', $legacyOption)) {
$legacyOption = [$legacyOption];
}
$newOptions = [];
$defaults = [
'additionalInformation' => null,
'action' => null,
'form' => null,
'method' => null,
'inputs' => [],
'logo' => null,
];
foreach ($legacyOption as $option) {
$option = array_merge($defaults, $option);
$newOption = new self();
$newOption->setCallToActionText($option['cta_text'])
->setAdditionalInformation($option['additionalInformation'])
->setAction($option['action'])
->setForm($option['form'])
->setInputs($option['inputs'])
->setLogo($option['logo']);
$newOptions[] = $newOption;
}
return $newOptions;
}
So as much as I get it, at the beginning of this
if (!$legacyOption) {
return;
}
if legacyoption is empty or null this method is called upon without an explicit return value.
So much for the theory. Then again, I'm no coder - even less one of the people that coded prestashop.
What I'm asking for is a solution for this: which settings do I have to set inside prestashop for this error to disappear?
Thank you in advance 
(annotation: Being a Servicedesk guy myself I can appreciate a "I've picked up your request and escalated it, please be paitient for X days until your issue can be adressed" ... )