Seems this might be a bug.
If you have 1 vol point EG <OptionTenors>1Y</OptionTenors> <SwapTenors> 10Y</SwapTenors> then everuthing works fine.
If you have a term structure as defined above then you get the error message but if you add say an 11Y option tenor then everything works fine.
IMHO I think that it should be ok to define just a swaption diagonale with a single tenor.
Again thanks for the tool.