Paypal Payment Failed Multiple Times

Hi there,
I have been using the currency switcher on my website since December but only with one currency Checkout enabled and everything worked normally.

I have 2 payment methods on my website: Paypal standard and Credit Card via 2Checkout. I only have problems with Paypal standard where all EUR payments are failing.

3 days ago, I enabled checkout with multiple currencies and Paypal payments started to fail. My default currency was USD but I noticed that three EUR payments yesterday and today via Paypal failed. Another EUR payment via Credit Card was fine.

I have now changed my default WC and checkout currency to EUR. I hope this fixes the problem but I wanted to find out if you have encountered this issue before? Can you tell me what I could be doing wrong?

Thanks for your anticipated help!