Hi Christoph,
If you are using versions of Compass older than 1.21 (which is currently in beta), the saved aggregations are stored in IndexedDB format in the local application preferences and are not easily accessible. In Compass 1.21, saved pipelines have moved to the filesystem (see: Where are saved aggregations in Compass? - #3 by Stennie_X).
However, Compass should not crash or hang when loading a saved aggregation pipeline.
Please file a bug report in the MongoDB Jira issue tracker with more details including your specific version of Compass and any error message or behaviour related to this problematic pipeline: http://jira.mongodb.org/browse/COMPASS.
Regards,
Stennie