Hi,
I have 3 Miktorik RB750gr3 to make a vpn.

Today i noticed some servers were down. I tried to log in to the main router, however it claimed i had to update winbox first. I have never seen this message before.

I looked through the settings and noticed the ip->firewall->nat table ist missing all my entries and now carries a huge number of stupid entries with ip addresses not existing in my network. (see screenshot)

The other routers seem unharmed.

What happened ???

Since those aren't dynamic entries, they have been added through admin interface. Most likely your router is compromised from leaving open ports to WAN interface. You should netinstall with latest version, use known good config and change all passwords.