Suddenly I can't host anymore

[Alexbra25]

I'm having problems hosting in SFD. My servers have always worked and people have always been able to join. But in the last few days, I started having this problem. People can't join my server anymore, it says "connecting" to them. I didn't change any internet configuration, I tried rebooting my modem and PC and nothing. The problem suddenly disappeared like yesterday, but it came back today.
Also, I don't know if it's related, but I'm getting an error (error 28) on the SFD launcher, saying that it can't verify my account.

[KliPeH]

I didn't change any internet configuration, I tried rebooting my modem and PC and nothing.

Some, if not most routers are set to change their configuration every while, or with every restart of the device. Either way the configuration probably changed whether you wanted this to happen or not. If you've been using the automatic port forwarding method, SFD created its own PF entry in your panel and now that the IPv4 address has changed both addresses are no longer in tune. Essentially, there's a disrepenacy between your actual IPv4 address and the one initially set in the entry, and so no forwarding of the ports actually happens.

I can only presume that's what happened from what little information you provided us with. Log into your router's control panel and update the automatic (or the manual, the process is the same for both methods) entry with your new address (found by pressing WinKey+R => "cmd" => "ipconfig").

The only way to avoid this in the future is to set up a static IPv4 address but that's not recommended for multiple reasons.

EDIT: Your friends aren't trying to use your old IP address by any chance, are they? Your IP address changed too and I can almost guarantee it. Make sure your friends are attempting to connect to the correct address and not necessarily to the saved/auto-completed one.

More possible solutions here.

[Noble]

I can confirm the same is happening to a lot of people, and to me as well, I wonder if the game changed something in this version? It's kinda weird how I'm sure my port fowarding is OK manually, static IP adress, and it was working so well and all the time before the update.

[Gurt]

We have not change how the port forwarding works. The Lidgren.Network library we use has stayed the same since Pre-Alpha. Port forwarding is entirely a local network setup we can't modify.
I guess people just tend to tune in shortly after each update just to discover that their port forwarding is no longer operational. I have never had any problem with my port forwarding but I also have a high end router for home networks.