I spent a whole hour trying to convince the check-in staff in Belgrade that I absolutely needed to be registered for the flight. They spent a whole hour convincing me otherwise.

Timatic apparently showed them that I needed a Singapore visa, which I didn't have. But on the government website it says you don't need one if you have a ticket to a third country within 96 hours of arrival (which I did).

There was quite a heated discussion with colleagues on-site, three calls with different airline departments, and a couple of times they said there was no chance without a visa and I should just go home.

Eventually we figured it out. They apologized profusely for the misunderstanding and issued me a boarding pass. No problems with Singapore immigration officers.

Singapore airport is awesome. The greenest one I've ever seen. I'll go explore the country since they let me in.