Error 500 using PHP 7.3.29 (ok on 7.2.2) and largish payload

[mmcloughlinpca]

The payload is 1278491 chars of json-encoded data.

It works fine with PHP 7.2.2 but not 7.3.29.

Latest composer and dependencies.

No errors in IIS / PHP log for this.

Any thoughts?

Kind regards

• Mike