If you try to place a ghost over an unapproved ghost you get an error message like you do when trying to place a ghost over a tree.

Why is this an issue? You can't place modular blueprints and have them overlap to make sure the spacing is right. A workaround here is to hold shift and force the blueprint. But creating a switch from an unapproved rail ghost has the same error and holding shift doesn't work here. The workaround for rails is to approve the ghosts which kinda defeats the purpose of this mod.

Yes, my problem is as same as you have.

I had reported the issue and the author has already been handling it.
https://mods.factorio.com/mod/ConstructionPlannerContinued/discussion/67246ab1801e65ec712445bd

Mod creator said still need some time for this mod to get more sample crashing to see if there is something strange going on within the engine itself.

Hello,

I don't think this is the same issue - this is the problem with the placeholder ghosts interfering. This is currently the limitation of how the mod is implemented. I had some idea on how I could maybe work around it (by changing the layer the placeholders are occupying), but haven't had a chance to test it yet.

I'll try to post a bit more detailed response to this thread when I get some time. :)

Best regards,
Branko