You mistake BLANK with NULL here.
Typically ECC tables won't allow NULL values, since ABAP doesn't have a NULL concept.
Instead it uses BLANK/DEFAULT values.
There are many really good reasons for not using NULLs - if this is news for you, you definitively should read up about it. Dealing with NULL values can lead to very annoying and unexpected effects in SQL.
For your data load this means, you would probably have to either place BLANK or respective DEFAULT values into your source files or you would have to perform a staged loading and replace the NULL values in the second step.
- Lars