The crash looks very similar to these bug reports:
- https://bugs.launchpad.net/synapse-project/+bug/1401909
- https://bugs.launchpad.net/synapse-project/+bug/1219314
The solution in both reports is to run Synapse like this:
GTK_IM_MODULE='' synapse
If this fixes it, you can update the command in the Main Menu editor tool (System → Preferences → Look and Feel) for when it is launched via the Applications menu and/or in Start-up Applications (System → Preferences → Personal) so it applies at start-up.