Export Plugin creates 1px images

Hey all, 

With my plugin (evlaa), I run some export to create online galeries. 
Unfortunately, There is a bug since the 11.4 version of Lightroom Classic and the expoorted images are only a 1px large gray square. 

Do you have any idea on what's going on?

Here is the code I'm using. 

local export_session = LrExportSession{
    photosToExport = { photo },
    exportSettings = {
      -- temp folder
      LR_export_destinationType ="tempFolder",
      LR_export_destinationPathPrefix = Utils.getTmpDir(),

      -- full size jpeg
      LR_format = "JPEG",  
      LR_export_colorSpace = "sRGB",  
      LR_jpeg_quality = 1,  
      LR_size_doConstrain = LR_size_doConstrain,
      LR_jpeg_useLimitSize = false,
      LR_size_doNotEnlarge = LR_size_doNotEnlarge,
      LR_size_maxWidth = LR_size_maxWidth,
      LR_size_maxHeight = LR_size_maxHeight,
      LR_size_resizeType = LR_size_resizeType,
      LR_size_units = LR_size_units,


      -- no watermark for delivery
      LR_useWatermark = LR_useWatermark,
      LR_watermarking_id = LR_watermarking_id
    }
  }


export_session:doExportOnCurrentTask()
  for i, rendition in export_session:renditions() do
    local success, pathOrMessage = rendition:waitForRender()
  ...

Thank yoou so much for yoour help 

Martin