On your WHMCS payment gateway, if you face issues like this
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Error: Object of class stdClass could not be converted to string in /home/yourdomain/whmcs.yourdomain.com/modules/gateways/aamarpay.php:309
Stack trace:
#0 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Module/AbstractModule.php(0): aamarpay_link(Array)
#1 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Module/Gateway.php(0): WHMCS\Module\AbstractModule->call('link', Array)
#2 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Invoice.php(0): WHMCS\Module\Gateway->call('link', Array)
#3 /home/yourdomain/whmcs.yourdomain.com/modules/addons/RSThemes/hooks.php(146): WHMCS\Invoice->getPaymentLink()
#4 /home/yourdomain/whmcs.yourdomain.com/templates/lagom2/invoicepdf.tpl(7): invoiceVars(Array)
#5 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Invoice.php(0): unknown()
#6 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Invoice.php(0): WHMCS\Invoice->pdfAddPage('invoicepdf.tpl', Array)
#7 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Mail/Entity/Invoice.php(0): WHMCS\Invoice->pdfInvoicePage()
#8 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Mail/Emailer.php(0): WHMCS\Mail\Entity\Invoice->getEntitySpecificMergeData(27754, Array)
#9 /home/yourdomain/whmcs.yourdomain.com/vendor/whmcs/whmcs-foundation/lib/Mail/Emailer.php(0): WHMCS\Mail\Emailer->prepare()
#10 /home/yourdomain/whmcs.yourdomain.com/includes/functions.php(0): WHMCS\Mail\Emailer->send()
#11 /home/yourdomain/whmcs.yourdomain.com/includes/orderfunctions.php(0): sendMessage('Invoice Created', 27754)
#12 /home/yourdomain/whmcs.yourdomain.com/cart.php(0): calcCartTotals(Object(WHMCS\User\Client), true, false)
#13 {main}
Then you will have to find jsondecode or $url_decode = json_decode line on your payment gateway file and add , TRUE with that line.
$url_decode = json_decode($result, TRUE);
It should solve your issue.