I’ve got the exact same issue.
First I could apply the watermark for 100 images at once. The watermark was applied to one image size only.
Now I want to use it for 2 image sizes and the ‘Internal Server Error 500’ shows up. When I get it down to about 20 images it starts working again. Could you fix this? My library contains over 1800 images… Applying the watermark by 20 means that I have to click through 94 pages of images…
thanks in advance,
Bart