So this is my screen right before the Synthesis (just in case I'm missing something)


Once I click the OK button the Synthesis animation plays but right before it's completed I get a black screen with the "An error has occured" message and get logget out. I have tried many times both yesterday and today and it's always the same result. I have a free youkai slot so that can't be the problem.

I also have 2 other kijimunas above level 80 and the synthesis always worked fine for them

That must be a fairly recent bug. Did the patch on Monday fix it perhaps? Otherwise best course of action is to submit a ticket. Seems like certain parts of the game gets broken sometimes after updates, like the Korean text and Kaga castle.

I just tried again and I still get the same thing :/
What do you mean submit a ticket?

Sorry, I meant contacting the developers. From website page Support > Contact Us > Bug Report