Upgrading to new PayPal payment method results in 500 error

-- HikaShop version -- : 5.1.1
-- Joomla version -- : 5.1.4
-- PHP version -- : 8.2.24
-- Browser(s) name and version -- : Firefox

We've recently upgraded a Hikashop system from J3 to J5.
We've upgraded HS to 5.1.1
However, it is still using the old Paypal payment method.

When going to the HS configuration page we see the notice:

Your PayPal payment method is obsolete and will soon be discontinued by PayPal. Please go in the System>Payment methods menu, click on the 'new' button and select the 'HikaShop PayPal Checkout payment plugin' to configure the PayPal Checkout payment method which is now replacing it. Once done and the new PayPal Checkout payment method is available on your website, disable the old PayPal payment method and this error message will disappear.

However, when going to the System>Payment methods screen and clicking new we get a 500 error.

The details displayed are:

Compile Error: Array and string offset access syntax with curly braces is no longer supported

in .../plugins/hikashoppayment/platron/PG_Signature.php:17

and the specific line of code is:

if ( $len == 0 || '/' == $path{$len-1} ) {

I tried reverting the server back to PHP 8.1.30 to see if that resolved the issue, but it didn't.

Do you have a fix for this error?