The server ran out of memory
What causes it
The server used more RAM than it was allowed.
How to fix it
Give it more RAM (raise the RAM setting), or lower the view-distance in server.properties.
Usual suspects: the pack as a whole rather than one mod; memory-leak-prone mods (the AllTheLeaks report in the log, if present, names them).
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log