I've had this issue off and on since I joined the game. It's an issue with moving ships from old versions to newer ones. I just salvaged some ships from ~2 years ago, and ran into this same problem.
I suggest the following:
0) BACK UP YOUR FILE. I'M NOT JOKING. DO IT.
1) Find out the last version you saved that file in.
2) Get a copy of starmade running the last 0.19x version (eg, if you opened it in 0.197.36, I'd open it in the last 0.197.51 version (the last 0.197 version). -NOTE: Before 2015, there was a different build system, so I'd just advise using every 5th-10th build.
3) Load the world, run /force_save in chat.
4) Get your starmade you used to do this to the last build of the next 0.19x (eg, following the previous example, you'd use the last 0.198 build: 0.198.485)
5) Repeat until you are at 0.198.485 This may take a while.
///BACKUP YOUR WORLD BETWEEN EACH VERSION CHANGE.
If your ships disappear on world load, revert the version change, and revert the world file to the most recent backup. Then, zip your blueprints folder's contents, and make blueprints for every ship. Make a new world, and load in every ship from the blueprint using admin load. Then, back up the world, and try the version change again. If it still doesn't work, you'll have to use smaller version steps. If you have this issue trying to load into the version directly after to working one, I'm sorry, there's no more help I can provide.
6) Now, change to the 0.199.172 build (this is the one I used), and load and /force_save.
7)Now, change to 199.432 (Current Version), and load up. It should load everything fine.