Merging of poly lines is crashing ArcMap. This issue has been happening since installing arcgis 10.3 with available patches. Consequently we had to revert back to 10.2. Any feedback will be appreciated.
Any other instances of crashing, like when you merge polygons, using union/intersect tool, conversion of KML?
What is the data storage type? Try working in File GDB. If you work in File GDB, does the issue still exist?
Try disabling the background geoprocessing option. (Geoprocessing > Geoprocessing Options > Background processing. Uncheck "Enable".)
Run the tool again. Is it still crashing?