I used to be able to replace a synth by dragging the plug in from the browser to the synth track where I'm trying to replace it. It appears to be disabled now. I can drag to create a new softsynth track, but not replace one. Don't know when this happened, possible after last X3 update, but not sure. But as I drag it, when the cursor gets to the track it doesn't work and nothing changes.
I haven't changed any settings, but I'm pretty sure this should be working as it used to.
 

 . . . so you're saying it's just the drag and drop - to replace a synth is no longer working.
 . . . just wondering if the "right click" mouse on the track - - - replace synth - - - is still working for you ?
That's the way I've always done it, never used the drag and drop to replace a synth.