Drain odor after bathroom renovation

Hello,

some time ago we had a bathroom renovation done. We previously had two sinks next to each other and now only one. The drain of the second sink was closed with a plug.
Now, as soon as we run water into the remaining sink, it smells like sewage. What also probably shouldn't happen: when I fill the sink with water and then open the drain, air bubbles come up. I think that's the main problem (I am absolutely no craftsman! Just an office guy...).
Can someone tell me what is wrong here and how I can fix it? I have already read something about ventilation valves that can help, but I wouldn't know where to put them or if that would actually help.
No other drain emits odors, only this one. The vent pipe to the roof is also clear.

I hope someone can help me.

Regards
Andreas