I just hit this issue for the very first time. This thread has been very useful - thanks to all who contributed and shared their solutions.
I usually start VEPro by clicking a file on my desktop. Today I had the Kontakt sample data cache death spiral...
I was able to get my slave instances back using the File>Open method described in this thread. When I did get my slave back open again, I found something interesting...
I could tell that previously my slave was getting stuck loading my Spitfire Albion 1 woodwinds. Once I was able to open my slave template again, I notice I left Kontakt5 Spitfire Albion 1 Hi Wood Legato enabled when I last shut down. That instrument was showing that it was not connected to any audio (I route 4 mic outs) and thus, Kontakt was failing to launch properly. (this makes sense as I started my Slave before opening my Master DAW template). Once I started my DAW and connected all my instances, Kontakt was happy again.
So I made sure all instruments were in disabled state, shut down, and started again using my desktop shortcut. It loaded fine.
Now, I'm sure I've loaded this same template with other instruments left enabled... Perhaps there's something particular about the actual Kontakt instrument that is left open? Maybe because these Kontakt instruments need to see the multiple outputs in order to load properly...? I've seen other Kontakt instruments that behave very badly (wont draw the GUI correctly or wont complete loading altogether) in VEP due to output routing before - usually drums with multple outputs like GetGoodDrums for example.