Render fail because of cross dissolve

I have a small video for displaying stills involving 4 Fusion compositions, the first 3 of which are almost identical with just media input and text changes. The media inputs are stills (jpegs). The fourth composition is slightly different as it involves a short lens flare video with a still of a logo (transparent png file) overlaid.
If I render these as they stand, it all works fine. But then I wanted to smooth out the transitions between the compositions and added a cross-dissolve transition to the compositions on the edit page. The ones between the fusion compositions that are almost identical work fine, but if I add a cross-dissolve between the third and fourth, even though it appears to work fine in preview, Resolve always throws up a 'Render job failed as the current clip could not be pcessed: The Fusion composition on the current frame or clip could not be processed successfully' error when I try to render them all just at the point where the render meets the fourth compostion. If I remove the cross dissolve between the 3rd and 4th composition, it will render without errors.
Does anyone know why this might be and what I can do to get round it?
If I render these as they stand, it all works fine. But then I wanted to smooth out the transitions between the compositions and added a cross-dissolve transition to the compositions on the edit page. The ones between the fusion compositions that are almost identical work fine, but if I add a cross-dissolve between the third and fourth, even though it appears to work fine in preview, Resolve always throws up a 'Render job failed as the current clip could not be pcessed: The Fusion composition on the current frame or clip could not be processed successfully' error when I try to render them all just at the point where the render meets the fourth compostion. If I remove the cross dissolve between the 3rd and 4th composition, it will render without errors.
Does anyone know why this might be and what I can do to get round it?