Please let me know what versions of Lightroom and the Juicebox-Pro plugin you are using.
If you are not already using the Juicebox-Pro v1.4.4.1 plugin then you might like to try upgrading it.
You can find the latest version inside the Juicebox-Pro download zip package.
Full instructions for downloading the latest version of Juicebox-Pro can be found here.
I have just tried watermarking images (with text and an image) in the 'Output Settings' control panel of the Juicebox-Pro v1.4.4.1 plugin in Lightroom v5.7.1 and Lightroom v6 and the gallery and images export fine with no problems.
Does the problem happen only with the Juicebox-Pro plugin or with other Web Engine Layout Styles too?
Maybe the problem lies with your source images. Perhaps you could upload one somewhere (a file sharing site such as Dropbox) so that I can try to replicate the problem with one of your own images. Thank you.