I have defined two endpoints. One for creating a UTxO at the script address with a Datum that contains the wallet's own PaymentPubKeyHash. And a second endpoint that spends such UTxOs from the script address. The validator only checks for equality between the two PaymentPubKeyHashes.

First endpoint:

one :: AsContractError e => Params -> Contract w s e ()
one Params{..} = do
    pkh <- ownPaymentPubKeyHash
    let dat = Stake PPDatum { owner = pkh }
        tx  = Constraints.mustPayToTheScript dat $ Ada.lovelaceValueOf amount
    ledgerTx <- submitTxConstraints validatorInstance tx
    void $ awaitTxConfirmed $ getCardanoTxId ledgerTx

Second endpoint:

second :: forall w s e. AsContractError e => Contract w s e ()
second = do 
    pkh   <- ownPaymentPubKeyHash
    utxos <- Map.filter (isSuitable pkh) <$> utxosAt validatorAddress
    ...
    where
        isSuitable :: PaymentPubKeyHash -> ChainIndexTxOut -> Bool
        isSuitable pkh o = case _ciTxOutDatum o of
            Left _          -> False
            Right (Datum e) -> case PlutusTx.fromBuiltinData e of
                Nothing -> False
                Just d  -> owner d == pkh

But the equality fails between the PaymentPubKeyHashes.

After testing around I recognized that the type of ownPaymentPubKeyHash does not just return a PaymentPubKeyHash but a Contract w s e PaymentPubKeyHash.

So how can I create the datum so that the PaymentPubKeyHashes match?