We are using PayPal express payments and the OneStepCheckout module.
A customer reported that he tried to make an order, but when clicking "buy" the waiting animation keeps being displayed and he is never being forwarded to the PayPal website.
When we tested this, we could not reproduce the problem.
After cleaning the Magento caches, the same customer was able to place an order (while it is not clear, if this is related).
What can be the reasons for this? How can the problem be troubleshooted?