I recently had trouble using PayPal’s Sandbox accounts with a couple of test credit card numbers that I found online. I was getting an Internal Error message from PayPal, a timeout of sorts. PayPal Support as very quick to explain that the numbers easily found online are outdated. Here are the numbers they gave me for testing. I hope they’re helpful to you as they were to me.
5355821617452825 4539231797097766 371679978113167 6011428784506930
5285570542915472 4929855217305330 375793778613841 6011529879410217
5383804521984816 4532402209297966 343237404492203 6011077434086858
5100568622823460 4929125342648081 346204752699380 6011316919072953
5174158963595579 4915612105806150 371496720560003 6011802923231073
5448506129042417 4929768483269256 343308938304568 6011472168799398
5254734479324127 4532882997894050 345021646614818 6011208593683884
5222972685589732 4532616565981810 345116149238519 6011209316853077
5115652007791949 4024007135023926 345682132459826 6011840879000860
5233352188033933 4556716213378921 376317928643562 6011228616237179