There is a bug in Subscribe by Email v2.4.6 (WordPress v3.6.1; MySQL v5.1.66-community on Microsoft-IIS/7.5)
In two locations in model.php there is SQL that attempts to set the default value of subscription_settings to ''; this results in a database error that a BLOB/TEXT column cannot have a default value.
This is reproducible when installing the plugin fresh, or upgrading (which triggers an alter table action) on MySQL 5.x on Windows (see more information here: http://stackoverflow.com/questions/3466872/why-cant-a-text-column-have-a-default-value-in-mysql)
By the MySQL 5.0 reference a BLOB/Text can't have a default value. On *nix this is only a warning (and ignored), but in Windows this is an error.
The solution is to remove the default from the two query's. I.E. find the text "subscription_settings text DEFAULT ''" change it to "subscription_settings text".
Please add this fix to the next revision. Thanks.