You should literally never update engine on projects you are working on, particularly when they work.
I hope you can revert back.
If you can, just buy the plugin, install a version of the engine compatibile with it. Create an new project, and then manually reproduce the plugin within your working engine version.
.27 is plagued by issues.
Another “patch” which could get the .27 version to run ok would be to grab the latest source of .27 and build it yourself.
I have to do that just to be able to use .27 currently. As the launcher version just bugs the UI out.
That said.
Your CAM_player_cam
Is what exactly?
What is the parent class? Is it custom cpp? Or just a piece of the engine which probably got updated from under you causing the problems?
The issue could be as simple to solve as to override the class cpp and h files with the old source code and build the engine from source (to just replace it).
Or, you can create a custom class in CPP, and feed it the same .h and .cpp that the engine of the working version uses.
Then switch the patent class in the BP to the one you created.
It all depends on what components were used really.
I would still revert back to the old version and work the plugin into it manually