I think there is absolutely no relationship between the SWR shown on the FT-817 and the SWR from the 2,4 GHz PA output.
Your problem seems to be between the FT-817 and the transverter input.
Cable connection flooded or something.
The dummy instead of the 2.4 GHz antenna must of course be suitable for the frequency and power.
But, as you write this, you have to look for the error between the FT-817 and the transverter anyway.
You can also connect a dummy instead of the transverter and watch the VSWR when it is wet.
73 de Robert
Thanks, Robert. Any suggestion where to buy or how to build a 2.4 ghz dummy load?
Makes sense that the problem was between the FT-817 and the transverter input, but all is working fine now. The cable connection between the FT-817 and the transverter is indoors and has not changed. The only thing that has changed is the weather -- sunny today, rain yesterday. If I can get a dummy load and it happens again, I'll do more testing and checking.