I have to admin there is slight difference between the columns from both tables. But I thought that was minor.
The table Source has three attributes
insert into DB_B.Target select * from DB_A.Source ;
SQL Error: ORA-01722: invalid number -- 01722. 00000 - "invalid number"
Maybe the order of the columns is not identical in the two tables.
Try to explicitely list the columns to make sure they aren't mixed up for some reason:
insert into db_b.target (amount, shortname, name) select amount, shortname, name from db_a.source;
Note that it's generally considered bad style to not list the columns in the insert clause and to use
select * in that way (even if that isn't the cause of your problem)