Marco,
I am Nicolas and I work with Andrew. I am the one currently maintaining our plugin. We did detect that the issue was coming from our plugin - or sort of - as it is working when you disable the plugin.
Currently, the only way to have our 64 bits plugin to work (built with Visual C++ 2012) is to run it thru a "relay" - a DLL I did build using Embarcadero C++ Builder Seattle (Trial) that act as a plugin, but relay the calls to our VC++ compiled DLL. Otherwise for some reason the plugin never load. (But no issue at all in 32 bits with the same code compiled...)
That relay expose and relay all functions that are available for a plugin. In that case, the function OnTemplate was the issue, as the relay return a null boolean in case it cannot find a function on the other side. I did expose the function in our plugin to return TRUE, and it work now.
Fidelfs - once PL/SQL respond again, try to switch window to something else and back to PL/SQL Developer. It did happen to me to that the window hide and loose focus, I was in need to do that to have it showing. Check also by disabling custom plugins, in case of one of them is causing trouble, as it did happen to us.