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I am getting the same issue in sandbox.
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Is this happening when making API calls and then redirecting to PayPal, or is this happening with PayPal Standard buttons, simply trying to login to PayPal, or when exactly? Have you tried clearing out your browser cache / cookies to see if that helps.
While I am doing post form data to sandbox that time that Internal Server Error. That error not occur every time some time it's happning and some time showing 500 error. After clearing clearing my browser cache / cookies it's work fine. But we can't tell our client to clear cache / cookies. So Please help me to overcome from this.
This is something that can happen a lot during testing because the sandbox is pretty finicky with these cookies, and when you're running a bunch of different tests it can be corrupted and cause these Internal Server Errors. Since your client isn't doing it nearly as much as you hopefully it wouldn't happen to them, but if it does you'll just need to ask them to clear the cache the same way you did and explain that it's something that comes up in the sandbox but shouldn't happen live.
Unfortunately, dealing with sandbox issues like this in PayPal is something you sort of have to learn to live with.