The script expects your image to look like this (which is in fact the output of a 'Shred execution:
So, the fencepost rule says that you will have one more strip than spaces. So if you want to cut 3px spaces between 10px strips, you image should be N×10+(N-1)×3 pixels, or , if you prefer, 10px for the first strip, plus some multiple of 13px (space+strip) for the rest. Here, 10 strips of 10px and 9 spaces of 3px, so 10×10 + 9×3 = 127px
In you case your are using 32px strips and 4ps spaces, so your image should be a number of the form 32+N×36: 32, 68, 104, .... , 500, 536, 572... . Since you have 535 your image should probably be 536, and may you shaved off an extra pixel in the workflow. So either you correct the problem upstream or you enlarge the image or layer, but you then have to wonder what to put in that extra pixel column (and also if the lost pixel is on the right or on the left).
Important note: while making the screenshot above I found that the script didn't work in Gimp 2.10 (previous update was in 2017...). I just uploaded a fixed version on SourceForge.
So, the fencepost rule says that you will have one more strip than spaces. So if you want to cut 3px spaces between 10px strips, you image should be N×10+(N-1)×3 pixels, or , if you prefer, 10px for the first strip, plus some multiple of 13px (space+strip) for the rest. Here, 10 strips of 10px and 9 spaces of 3px, so 10×10 + 9×3 = 127px
In you case your are using 32px strips and 4ps spaces, so your image should be a number of the form 32+N×36: 32, 68, 104, .... , 500, 536, 572... . Since you have 535 your image should probably be 536, and may you shaved off an extra pixel in the workflow. So either you correct the problem upstream or you enlarge the image or layer, but you then have to wonder what to put in that extra pixel column (and also if the lost pixel is on the right or on the left).
Important note: while making the screenshot above I found that the script didn't work in Gimp 2.10 (previous update was in 2017...). I just uploaded a fixed version on SourceForge.