[Plugin: Contact Form 7] No action when switching to customized permalinks

This Problem occured when I switched from default permalinks to customized permalinks. What can I check or change? Default permalinks are no longer an option..

The spinning arrow icon now shows up once I submit a form, but then nothing happens. I have tried deactivating all other Plugins but nothing changed. There is still just the spinning arrow.

Thanks for your great plugin and your support!