Paypal Standard processing error

I'm getting customers telling me that their payments are not working. Paypal is displaying an error...

An error occurred while processing your request. Before submitting this again, log in and check your Account History to ensure that the transaction didn't complete.

What is causing this and what is the fix?

(see attachment)

Sorry to bring this issue back to the forefront, but I am still getting customers receiving this error! Certainly I can't be the only one. I'm at a complete loss as to how this suddenly started happening despite not making a single change to anything on my store.

Im getting something similar. after years of it working just fine suddenly its not accepting payments anylonger. Paypal is absolutely NO help.