Lars - not sure if you have seen this but this is a common problem if you export tables from ECC to a flat file and try to load them into a HANA schema which has taken the tables from another HANA side-car environment.
With SLT you don't get this issue.
In this case, you can end up with "null" values in the export, which then cause this problem on the import.
The only solution I know is to use Data Services to do the load rather than flat files, and to do the mappings with a default value.