Check if there are objects attached to the cable that are being affected by the spring forces. If there are such then block time warp since activating it will result in unpredictable behavior (most likely an explosion or collision).

Alternatively, make such objects still before going on rails.

Some research background. KSP destroys any non-active and not landed/splashed vessel. Since hanging vessel is considered to be in flight it will explode on time warp. The only way to avoid it is faking hanging vessel state. Though, it would raise a lot of persistence and compatibility issues.

Better approach would be restricting saving when any vessel is hanging. Though, this may raise a question how to deal with in-orbit vessels.