@Erpel @kaia
I usually observe something like this when my ISP gets DDoSed and they start dropping incoming packets to the externally reachable IP-address they provide me with: I can see what everyone's posting as my instance can access the Internet, but no one can see my replies — as their instances have to retrieve them from mine, but they can't do it.
Probably there's something similar in your case — your instances have only one-way connectivity.