Hi Julian,
And sorry for the late reply – we just temporarily limited the support and focused on development, that’s why we have some delays.
When it comes to your case:
1. Try bulk watermarking smaller number of files at once. There are might be different server limitations that make it impossible to process larger number of files at once. There’s not much you can do at the WP level (but try to increase memory limit in wp config). I suggest asking your hosting provider for some help and guidance with the error.
2. Image regeneration plugins are useful if you want to remove/change the previously applied watermark from your images. It uses full size image (the original one) to re-generate other image sizes, so the best practice is to never apply watermark (or any other image modification) to those full size images. If you didn’t apply watermark yet, it will do nothing for you.
Hope this helps.