I have my own server setup. Occasionally I can open a door, but cannot walk through it. Also blocks will not be destroyed. I have found that logging off then on does not fix it. Changing form does not work either. I did cure it by creating a new folder, adding fresh server files, and running the launch.bat, then putting the old world into the fresh server folder.
Can't work out what triggers the error, but it seems to be soon after I have logged back in.
Great modpack guys, really enjoying it. A lava shortage is my only gameplay issue at the moment . Bat mode for flying around is cool, and squid mode for finding clay is handy.