You can grab PMS version 1.14.1.5488 from How to download plex 1.14. You didn’t mention what architecture you need, so I hope this one works. Version 15 is having issues for everyone not just people running Ubuntu 14.04. You can check out this thread to see if this is the same issue you are having Ubuntu 18.04.2 LTS - Plex Server Crashes - Double free or corruption (!prev)
