Was just starting up doing stuff on ymstcraft in my modpack was placing down blocks and i crashed with the following report.
crash-2016-10-30_20.34.58-server.txt

Isolated issue to clicking on writing desks, moving to mystcraft-issues.

Yup! Known issue. Sorry about that.
In the future, please pastebin the log (or similar) rather than upload it as a file. It is much easier to review issues in a browser if I don't need to download anything. :)