I am running Zen Cart 1.51. I downloaded the latest version last week.

After installing TaxCloud my script log showed that the TaxCloud modified file,
admin/includes/functions.general.php was causing an error. The path on line 11 was incorrectly specified.

The error in the script log was:
PHP Warning: require_once(../includes/modules/TaxCloud/func.taxcloud.php) [<a href='function.require-once'>function.require-once</a>]: failed to open stream: No such file or directory in /[store path]/azcadmn/includes/functions/general.php on line 11

This line in general.php caused the error:
$taxcloud = '../includes/modules/TaxCloud/func.taxcloud.php';
require_once($taxcloud);

I changed both lines (above) to one line (as shown below). The error is gone and it's working fine.
require_once(DIR_FS_CATALOG . DIR_WS_MODULES . 'TaxCloud/func.taxcloud.php');

TaxCloud seemed to be working without this fix (and may have caused a future problem). It did cause a problem with a Zen Cart QuickBooks Sync product add-on. The error caused the add-on not to work.

I reported this error on the TaxCloud site, but thought I should post here too.

Hope this info can help others.