Redmine

Could you provide images that cause the problem?

I also experienced such issue when using ckeditor and pasting the image with CTRL-V.
The images will be populated like:

<img src="https://1.2.3.4/attachments/download/1104">

the function get_image_filename looks like this in 5.0.5:

        def get_image_filename(attrname)
          atta = RDMPdfEncoding.attach(@attachments, attrname, "UTF-8")
          if atta
            return atta.diskfile
          # rubocop:disable Lint/DuplicateBranch
          elsif %r{/attachments/download/(?<id>[^/]+)/} =~ attrname and
                atta = @attachments.find{|a| a.id.to_s == id} and
                atta.readable? and atta.visible?
            return atta.diskfile
          # rubocop:enable Lint/DuplicateBranch
          elsif %r{/attachments/thumbnail/(?<id>[^/]+)/(?<size>\d+)} =~ attrname and
                atta = @attachments.find{|a| a.id.to_s == id} and
                atta.readable? and atta.visible?
            return atta.thumbnail(size: size)
          else
            return nil
          end
        end

as you can see the line:

elsif %r{/attachments/download/(?<id>[^/]+)/} =~ attrname and

is checking for a trailing "/" (slash) which my redmine does not insert!
-> the issue occures.

To fix this i have changed the line to:

elsif %r{/attachments/download/(?<id>[^/]+)} =~ attrname and

Just removed the trailing slash.
And now it works just fine.

Here in beyond compare: https://imgur.com/a/3nEVTtb