I was able to reproduce this error again. Here is what I did:
I created a new table
I created a new field in that table (however, unbeknownst to me the field name that I tried to use is a reserved word and therefore was not able to be added.
I then tried to insert records into the new table (obviously not going to work because the field could not be added to it). When I attempted to insert the records I got the error "It does not appear the field Day exists in the table."No problem here.
Now when the program tried to continue adding fields to the customer table I got the following error each time:
Failure. The API could not validate your log in. Please make sure you have entered the correct Company Account, User and Password, and that each are spelt correctly. Also, ensure that this User has been configured for Method API access by going to Customize > Users, editing the User, and ensuring that there is a check in the 'Is Allowed to Connect via MethodAPI' checkbox.
Hope that helps you to be able to track the issue down.
~ Method user / partner since 2008
~ Over 15 years field service experience
~ 2013 Method Partner of the Year
~ Degree in Computer Science