I should also note that, unfortunately, there is a known issue whereby the error can be generated if the resize image dimensions are too large. This might be the reason for your problem.
The developers are aware of the problem and it has been logged as a bug so it should hopefully be addressed for a future version.
In the meantime, the only workaround is to reduce the resize dimensions until the error is no longer displayed.
I realise that this is not an ideal solution but, until the bug is fixed, there is no way around it (at least using JuiceboxBuilder-Pro).
I do not know the exact nature of the problem (why large resize image dimensions cause the error message to be displayed), but, having experimented with large images myself, the error seems to be triggered when the Max Width and Max Height both exceed a threshold value somewhere in the range of between 2364 and 2504.
If you need to create a gallery with images of a size that triggers the error, then the best course of action might be to do the following:
(1) Create a gallery with JuiceboxBuilder-Pro, using resize image dimensions that do not generate the error. This will at least generate a 'config.xml' file containing the imageURL entries.
(2) Create a set of images in a separate imaging program (such as Adobe Photoshop) at the required size and then copy them into the 'images' folder inside the gallery folder (overwriting the images originally generated by JuiceboxBuilder-Pro). Just make sure that the image filenames match the imageURL entries in the gallery's 'config.xml' file.
I understand that this adds extra steps to your workflow (and should not be necessary) but, until the bug is fixed, it might be the best option.
I hope this this at least helps to explain what you are seeing and provides you with a workaround.