On the confirm page, there are two server behaviors to store order information in the data base:
eCart Store order Summary and eCart store Order details.
the error is probably happening in the eCart Store Order Summary behavior.
Either the value OrderAmount is trying to be stored in a numeric field, or the store order summary is trying to use a field named order amount that does not exist in the database.
Your host should have a WebSite control panel that will give you access to the MySQL database using PHPMyAdmin. Double check in the orders table for a column named OrderAmount.
I tried registering a test user, and letting the session lime out, then loging in again, I cannot reproduce the issue with the login failing.
is this an issue you can reproduce? we will need to be able to reproduce the problem to be able to determine what is causing it.