I can confirm from the error log that the PHP installation has not been compiled with Argon2 support. Argon2 is important for my site to maintain compatibility with a specific data format.
This forum post states the inclusion of support for Argon2 in InfinityFree: https://forum.infinityfree.com/t/php-7-3-is-now-available/17393
Could it be a misconfiguration with this particular account? Is it possible to get support for Argon2?