Hi Suting,
I've checked the models (no.18-22) you uploaded. An image of model no. 20 is attached, which shows clearly that G4, G5, and C26 are split apart. Especially, G4 can no longer be taken as a purine due to incomplete base ring atoms, which explains why the x3dna_ensemble script stops at this entry. Similar issues happen with models #19 and #22, whilst models #18 and #21 look fine. Please double check your procedure on how the MODEL/ENDMDL-delinated ensemble of MD models are generated.
In this case, as in several scenarios I am aware of, 3DNA is helpful in identifying inconsistencies in the analyzed dataset. Care must be taken to ensure sensible results in an otherwise complicated process/pipeline.
HTH,
Xiang-Jun