"Transaction not found" when Capturing payment transaction with Bambora

I'm implementing the Inline Checkout from Bambora. The authorization step works without any problem. But when it comes to capturing the payment I always get the error Transaction not found.

This is what I do:

const options = {
    headers: { Authorization: `Basic ${ apiKey }` },
};
const { payload } = {
    amount
};
const { data } = await axios.post(`https://transaction-v1.api-eu.bambora.com/transactions/${ txnid }/capture`, payload, options);

I also tried making the request with included transactionoperations:

const options = {
    headers: { Authorization: `Basic ${ apiKey }` },
};
const { payload } = {
    amount,
    transactionoperations: [{ id: txnid }],
};
const { data } = await axios.post(`https://transaction-v1.api-eu.bambora.com/transactions/${ txnid }/capture`, payload, options);

I get the same error when trying to get transaction information:

const options = {
    headers: { Authorization: `Basic ${ apiKey }` },
};
const { data } = await axios.get(`https://merchant-v1.api-eu.bambora.com/transactions/${ txnid }`, options);

As I understand the documentation I'm doing everything right, but I can't get it to work.

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