Sorry — your last action could not be completed

[epgb101]

I keep getting this error randomly when I click/test my Paypal Donate button. Then next time I click I get the donate page. Then next time 

"Sorry — your last action could not be completed" page

Anyone know whats causing this? Members of my site are always complaining they cannot pay me. Yet I get some paypmnts fine.

Thanks

Ed