Having a white background rectangle helped!

Weird, I tried the pattern approach and have the same issue - the lines where the stroke of the new object would have been are visible in PDFs...

OK, I'm wondering whether it's a regression in the newer version of Inkscape then, thanks.

Are you saying it works for you with no stroke? I have tried it, I still get that weird line in the pdf. What version of Inkscape are you using?

Thanks everyone for the suggestions.
druban, please find svg, png and pdf (printed png) files here: https://yadi.sk/d/Uon26JECsNKWs The svg file has one bitmap image in it with a clip. Would be great if you could figure it out.

Thanks, that's a good idea! Would save some time. I am really not that familiar with Gimp, try to use Inkscape whenever possible.

Thanks, I'll have a look at the extension.

Well, I can, it's just I have already done everything in Inkscape and wanted to simply export what I have. If I have to, I'll do it again in Gimp of course...

No filters, and it doesn't help
Also, just in case, the clip is not simply a rectangle, but a more complicated shape.

I'm exporting a selection, and I hide all non-selected objects. Do you think it can make a difference? I tried copying the selection to a new document and exporting Drawing, I got the same artefact...

So, should the bitmap be aligned to pixels grid in Inkscape? But what if it has a totally different resolution from my inkscape document? I tried snapping both the bitmap and the path to the pixel grid, but nothing really changed...

I have a problem with clipping a bitmap in Inkscape. After I export a clipped bitmap into PNG it looks fine. However after I print this PNG into a PDF file, insert it into LibreOffice writer (and save it into a PDF from there as well) I get a thin grey line where my clip path in Inkscape used to be....