After a HTML export from a page images are missing (broken link symbol in HTML page) under following circumstances:

• The image contains a "+" sign or a space (" ") in its name and
• the image is not attached to the page itself, but is referenced from the exported page but is attached to another page (not being exported)

• Please also note that images containing a "#" character in their file name are never exported to HTML, even if they are attached to the exported page itself.

We are using the default "Space export" functionality, not any 3rd party plugin.

See the attached screenshot: This is a HTML page exported from confluence. The upper image was named "Warnmeldung Wasserrecht.png", the lower simply "Warnmeldung.png". Both images were attached to a different page not included in export. The lower one appears correctly, the upper one doesn't.
The missing image is actually missing from the "attachments" subfolder in the created HMTL.zip file, that's why it cannot be shown.

I was able to create a test case within 5 minutes, so this should be easy to reproduce.

The only "workaround" is extremely time consuming, because it basically means to re-organize and rename many of the images of every page and rework the pages accordingly. Hardly a solution for a whole space. I therefore consider a true fix for this absolutely necessary.

Note: Together with bug CONF-31714 this renders Confluence rather useless for creating (software) documentation which needs to be exported, at least in our case where we use (and re-use) lots of images on the pages. This bug has cost us 5hrs already + 1hr for the bug report.