309 days late but did you ever solve this
I had the same mess with Leaseweb in 2022. Their billing portal also died with a 500.
What actually worked was requesting a chargeback reversal letter from my bank. Sounds backwards but the dispute team had the full merchant descriptor including invoice number. Cost me a phone call.
Never got the actual PDF though. Tax office accepted the bank letter with the PayPal screenshot attached. YMMV depending on country.
Still curious if Hetzner ever came back to anyone. Their status page still says "operational" which is a joke.