Up until a few hours ago, my VirtualBox setup worked perfectly, Windows 10 Host, Ubuntu 15.04 Server and Linux Mint 17.2 guests. Both guests set to bridged networking mode and every device on my home network could talk with every other one (guests with the router, host with the guests etc.).
My router/modem is a FRITZ!Box 6360 Cable, issued by my ISP, with its default DHCP settings and no additional settings like client isolation etc. set.
Without any deliberate change in configurations, suddenly the communication between my host machine and the guest machines does not work anymore. They can still both talk with everything else and have working IPs. None of my machines or network equipment block pings.
I already reinstalled my VirtualBox and spun up a new Ubuntu 15.04 Server VM, but with no different results. The communication does work with the Host-Only network setting, but I would much prefer bridged mode, to be able to access the machine from other ones easily as well and I cannot fathom why it suddenly would stop working.
What could be the reason for this behaviour and are there any ways to fix it?