I am using dbunit to create database backups, which can be imported and exported. My application can use several database engines: MySQL, PostgreSQL, SQLServer, H2 and Oracl
I had the same AmbiguousTableNameException while executing Dbunits aginst Oracle DB. It was working fine and started throwing error one day.
Rootcause: while calling a stored procedure, it got modified by mistake to lower case. When changed to upper case it stared working.
I could solve this also by setting the shema name to IDatabaseTester like iDatabaseTester.setSchema("SCHEMANAMEINCAPS")
Also please make sure your connection doesn't access only to many schemas having same table name.