Looping means that participant culdn't advance from the welcome page to the 1st question

I wasn't able to reproduce the looping situation.

I understand the issue of having the dropdown with no value, although on the DB there is a value.
To fix this, the value saved on DB while installing should be "themes/survey/fruity/files/logo.png".
Also an update script should be done for replacing "./files/logo.png" with the correct value ofr current installations.
Still, I think that's minor and not the objective of this ticket.

Let's review again.

Thanks

I think the biggest problem here is that the file is saved with a path, which makes no sense because paths can change, also the usage of the logo is not the file path but a URL.

The theme could be renamed, or the path could change in general if the whole upload folder is moved.

The beginning of both paths need to be interpreted as placeholders ( either starting with \themes\survey or \upload\themes\survey\ ) and both would need to be relative URLs (relative to the LimeSurvey root URL)

THis will be a constant nuisance unless fixed properly.

Current default for a core theme is
./files/logo.png

but for extended themes a full path is saved, which is unnecessary and also probably unsafe.

How about the file is always saved relative the way the default is?
If the theme is extended just use the theme folder.
Most problematic part is the update of existing paths to the correct scheme.

This happen between 2 minor release :

User add svg in allowed file upload
User use svg as logo via css.

in a twig file
<style>
.logo-banner{
background-image: url ( {{ imageSrc("./files.mylogo.svg") }});
}
</style>
Update to last version :
https://github.com/LimeSurvey/LimeSurvey/commit/756c1687ab30cebe8849aa9ca9ffda79cd6ed647
Same style give
<style>
.logo-banner{
background-image: url ( );
}
</style>

A call to https://survey.example.org/index.php?r=survey/index&sid=217451&newtest=Y or https://survey.example.org/index.php?r=survey/index&sid=217451
Expression manager start again.

click on next : no effect : coma back to welcome page.

How about the file is always saved relative the way the default is?

The file is not saved with relative path ?

It's OK using image but not imageSrc currentmly.

The issue is <img src=""> make a call to current url …

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=31432

Fix committed to 3.x-LTS branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=31433

Extra enhancement for avoiding path traversing...
For Your Reviewal
https://github.com/LimeSurvey/LimeSurvey/pull/1927

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=32819

Fixed in Release 5.1.18+211101