Thank you for your patience in dealing with this issue - it really is appreciated and a demonstration of superb support. Many thanks.
Quite welcome. 
I did some investigation into the ARI issue related to your certificates, but was unable to proceed due to data changing on the Let's Encrypt side. I think the ARI issue might have been being caused by a temporary issue on the Let's Encrypt side. You might try switching back to the stock version of CertSage 3.3.1 to see if the "malformed" error persists.
Try acquiring a new cert (using the modified certsage.php) then see if that still happens with the stock version afterwards. I think your current cert was issued during that weird period.
Successfully acquired and installed new certificate under the 'no ARI' version, but got the same 'malformed' error when I reverted to the stock version.
It may take a bit for the ARI information to come through on the LE side. If it's still showing that error after a couple of hours then there's a concern. I'm finding it very strange that this has not happened with any of my domain names or your other domain names.
May be worthwhile to check for errors similarly to that one?
This is in the error logs:
[07-Jun-2026 11:24:52 UTC] PHP Notice: Undefined index: serialNumberHex in /home/juolnmby/public_html/shawcottage.farm/certsage.php on line 679
I haven't upgraded any of my other domains to 3.3.1
Yep. There's the problem. @MikeMcQ messaged me as such. The version of PHP being used by LiteSpeed to serve shawcottage.farm (5.6.40) is too old and thus doesn't include that necessary index for ARI. This might also explain why you can't use EC. CertSage has always required PHP version 7.0+.
Check to see if you can up the version of PHP to at least 7.1.2. This type of thing (though usually for Apache) might apply to your situation in terms of what's holding back your PHP version for shawcottage.farm:
Is it possible to update CertSage to check for the PHP version, and if it is too old, report and stop doing further action?
The server, and all my other sites are running PHP 8.1 but shawcottage.farm appears to be running 5.6.40 (according to phpInfo())! Very odd - I'm checking now...
[juolnmby@premium702 ~]$ php -v
PHP 8.1.34 (cli) (built: Apr 20 2026 20:24:49) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.34, Copyright (c) Zend Technologies
with Zend OPcache v8.1.34, Copyright (c), by Zend Technologies
[juolnmby@premium702 ~]$
I think something like the last part of my previous post may apply here.
Yep. I should add a version check.
Found a hidden .htaccess file specifying PHP 5.6!! Now corrected and will try a clean stock version of Certsage...
Yep. That's what I was meaning. 
Huzzah! 
It's obvious - with hindsight (aaarghhh)!! Without your help and persistence I wouldn't have got there, so thank you once again.