Hey Maxzerr, It looks like you have hit one of the engine issues around editing structs. This is known and an issue has been raised and can be found here: Unreal Engine Issues and Bug Tracker (UE-40050) . To help get this fixed sooner, it would be great if anyone who has seen this issue votes on this ticket so that its priority can be raised.
You can find some additional information on when this occurs here: Modifying a struct involved in another struct breaks all functionnalites surrounding it - Programming & Scripting - Epic Developer Community Forums
As for a fix, there have been a number of things you can try which are mentioned in the additional information above, however they are not completely reliable. The best bet at this stage would be to revert the changes and fix any issues that occur in the blueprints. From there, create a project backup and then step by step make the changes to the struct, save, and restart the editor to see what state it is in. Personally I have only see this occur when editing sub-structs, so if this is also how you have made the changes to the main Card_Struct then you could also try simply adding the variable to the main struct instead of a child, and hopefully should be able to by-pass this issue.