After completing the payment screen and clicking 'Confirm Contribution' the screen hangs. The button is greyed out but nothing happens. But...if I just hit enter on the screen without first completing anything at all I get an error message about incomplete fields, and then I can complete the screen & proceed normally. But if I first complete all the details and then hit the 'Confirm Contribution' it hangs. I've tried this on Chrome, Firefox and on my Android 8.1 phone - all the same. I am using 5.2 of the Stripe extension, CiviCRM 5.7.3 running under Drupal. Using the second technique described above I've made a couple of payments so there's nothing wrong with the setup, it seems. Update: I have found some errors in the Stripe dashboard, here are screenshots and this
The answer is that I had pasted one of the Stripe API keys with a trailing blank, so when I looked at Firefox's web developer console there was a message 'Your key is invalid as it contains whitespace' Working consistently now, but how I previous managed to make a couple of contributions successfully I don't understand!