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This doesn't look right. Line 43 is just an opening brace. Moreover, there is no string access syntax using curly braces in Kickstart 7.1.2. This has been addressed years ago, when PHP 8.0 was just released -- that was the first version to disallow curly brace string index access. I checked a freshly downloaded copy of Kickstart Core 7.1.2 and it does not have anything like that.

Please download Kickstart Core 7.1.2 afresh. Extract it and upload the kickstart.php file.

Lines 42 to 45 should read:

if (!defined('KSDEBUG') && isset($_SERVER) && isset($_SERVER['HTTP_HOST']) && (strpos($_SERVER['HTTP_HOST'], 'local.web') !== false))
{
	define('KSDEBUG', 1);
}

There is no curly brace string offset access there, therefore you cannot receive an error about something we don't do at a code location which doesn't even have executable code.

Ask them if they have enabled OPcache, possibly in a way that caching does not take into account the path of the file (because that's the default setting, unless someone bothered to change it). This could explain why you get an error about something not happening in this file — it would be coming from what is happening in a different file by the same name, either an old version you have already replaced or possibly on a different account.

One way to find out is to renamed kickstart.php to something like kick20230206.php and try running that instead. If it magically works the problem is with OPcache.

Make sure that you both download and upload the file in binary mode. SFTP is always binary. FTP is not; you usually have to select it explicitly in your FTP client. Do not use the hosting control panel's file manager, they tend to not do very well with large files.

If your backup archive consists of multiple parts i.e. there are files with the same name but extensions .jpa, .j01, .j02 etc remember to upload all of them. They are parts of the same backup archive and all of them must be present for the archive to extract.