When trying to add credits (TF2 keys) I'm stopped by a warning asked to enable Steam Mobile Authenticator and per-trade confirmation, even thought that has always been enabled and I didn't change any of that. Still, I checked and it was indeed enabled as usual. What can I do? Thanks!

Maybe Steam was down? Retry and see if the issue persists.

By the way, I'm also able to trade with other people normally, not going through escrow or anything. Please help me, I need to deposit 200 TF2 keys as credit, I could do it in other way if I must. Thanks!

Depositing keys. I was trading with dispenser bots right before I needed credits.

I believe there was an option of mobile authenticator about "manual verify trade each time" and you have to enable it. It seems there is problem for you sending away items but not receiving them.

Is your account fully capable of trading without escrow? Any recent security setting changes?

I cannot think of the reason as you are the only person having this problem.

Nice, that's good to hear. :)