Chiming in -- I began having this issue today after switching from the FTB Unleashed beta to the recently released FTB Unleashed version. I enabled a few of the disabled mods (like Biomes O' Plenty) and booted up a new server. I couldn't get more than a few seconds in before this issue started.
I added tekkyjim's suggestion to my startserver.bat and the server is working fine.
StartServer.bat
java -Xms1024M -Xmx2G -XX:MaxPermSize=256M -jar ftbserver.jar