Invalid JSON string when saving a order

lately we have the problem when a customer places a order and this order is payed by card (mollie / Ideal)
And we want te edit the order we get the error: „youre trying to decode an invalid JSON String: Service Unavailable“
See picture

Shopware version: 5.5.8
Mollie plugin: 1.5.1
This does not happen with every order, whith our test account this never happens.

Does anyone have an idea what this may be?
I have search the internet and this forum, bur not find something concrete.