We have one project where if we attempt to copy the cost plan we get a 500 error. This was not occurring before our upgrade to 15.3. We are able to still copy cost plans for other projects.
The problem was caused by a file that was attached to the cost plan (via a custom field added to the properties page on the cost plan) not existing in the filestore. Most likely the issue was due to the fact that an upgrade and move to new servers for PPM took place two weeks back, and its possible the filestore was not copied to the proper path.
After removing the file from the cost plan we are now able to copy and save it without an issue. Additionally, check the filestore path in CSA/NSA to make sure that the directory exists and is pointed to the proper location where the files are actually located.