Upvote:1
Very difficult case (also because it is not clear what was the problem).
For next time, I would check job in Indonesia, to see what you can get, and so to have realistic expectation (which you can explain to the officer). Having some documents about your PhD. Is it a monograph or a series of published articles? Get some documents to show you are in good way to finish it. (as PhD on interesting topic, you get "fast tracks" to green card in US, so it would also show up that illegal immigration is not on your interest).
But prepare also plan B. If you will not get a dream job in Indonesia, what you will do? (not being sure is OK, but having some ideas and possibilities are nice, to show wiliness to return). Do you think you will get some job in Germany? Or a faculty position? [As you know, there is also the miss-understanding that PhD is just one step of academic carer, not as a starting point for a professional carer..]
Check if you can prove better your connections to Indonesia.
Do you have good explanation about the non-scientific conference? Again: this doesn't need to be very rational, but you should show that it is not just a random excuse.
Finally: when you discussed you PhD topic on interviewer, maybe you get on "easy mode" (the explanation you do to your parents and relatives), which could be seen as "fake PhD". You should not have problem on going deep (maybe ask some feedback). They will probably not understand, but this is not their task (to understand). They should get feeling from you that you are genuinely telling the true (and not repeating a story from memory).