Nicolas,
Thank you for your suggestions.
Unfortunately I am still coming up empty.
The log file path is writable. The reported file path to the log did not exist, as there was no 'logs' directory. After testing and failing a few more times, I created the 'logs' folder, and still POST fails but no log content.
/home/ip3inc/public_html/media/com_hikashop/upload/safe/logs/
I could not locate any language filter plugin, either in the Hikashop or the Joomla plugin manager.
I experimented with the Authorize.net hash value, still with no success.
To confirm use of the Authorize.net MD5 Hash Feature; the value entered in the Authorize.net merchant account settings, and the value entered in the Hikashop Payment plugin 'Your MD5 Hash on Authorize.net' are the same; ie plaintext values?
Your assistance is greatly appreciated.