Magento – magento PayPal gateway has rejected request. Security header is not valid (#10002: Security error)

I had this problem. Can't explain it, but it was solved by simply removing the paypal credentials and disabling the solution. Then entering the credentials again and enabling.

Worked fine again after that.

Paypal does not support all currencies.
You can find a list of supported currencies in Mage_Paypal_Model_Config::$_supportedCurrencyCodes.

'AUD', 'CAD', 'CZK', 'DKK', 'EUR', 'HKD', 'HUF', 'ILS', 'JPY', 'MXN', 'NOK', 'NZD', 'PLN', 'GBP', 'SGD', 'SEK', 'CHF', 'USD', 'TWD', 'THB'

You can make your base currency one of the supported currencies, enter your prices in the supported currency, fill in a conversion rate and use on frontend only your currency. Not a pretty solution but it should work.