Give the latest update a try (4.0.1) and see if the issue is resolved.

This is in 1.6.4 also... is there the possibility of an update to fix this?
I understand 4.0 onwards is 1.72 + but my server cannot be upgraded until all mods support 1.7+ - I'm sure im not the only one in that situation.

This issue doesn't just cause a server crash. It causes the server to crash immediately on restart until The mod is removed - making it useless from the point a player tries to divorce an NPC.

I use MCPC+ 1.6.4 B251 as the server core - if that impacts on it.

I understand that this issue is frustrating, but unfortunately I can no longer support 1.6.4.