They can't see any error on their end, despite the game saying it's an "internal error"?
I'm having the same problem right now. Made an offer to buy some zen, it for whatever reason didn't go through, and now the offer refuses to cancel. What's more, since it happened, the Dil Exchange won't work for me altogether - I can fill out an offer, then when I submit it, nothing happens.
It's pretty pathetic customer service to essentially say "Welp, you're screwed. I suggest checking your inventoty and mail (seriously??)." I buy a ton of dilithium, and if they can't fix this extremely soon, I may just be done. I'm not going to continue playing a hobbled experience.