Adding a watermark to image pdfs fails

[MichlF]

Issue: Adding a watermark to a .pdf document fails if the .pdf is an image without text. This occurs for images that were printed to .pdf with Microsoft print in Windows 10 as well as images converted to a .pdf using the web-pdf-toolbox function "IMG to PDF". The watermark function stops with an error and I can only download a document that's a blank page. To emphasis: it works flawlessly with a text .pdf (it may even contain a photo).

Error message:

This can take some time, be patient...
Processing...
This+is+a+test%21
This is a test!
Error: /undefinedfilename in (watermarked.pdf) Operand stack: Execution stack: %interp_exit .runexec2 --nostringval-- --nostringval-- --nostringval-- 2 %stopped_push --nostringval-- --nostringval-- --nostringval-- false 1 %stopped_push Dictionary stack: --dict:739/1123(ro)(G)-- --dict:0/20(G)-- --dict:75/200(L)-- Current allocation mode is local Last OS error: No such file or directory convert_pdf_compressed_pdf_watermarked_compressed.pdf Processing ok.

Platform: Selfhosted on a Synology disc station running locally within a docker using this image.