This is very complicated, but we have a fix.
Will post on Monday

Master: https://github.com/LimeSurvey/LimeSurvey/pull/3521

The patch is only for the source of the problem, which is when a theme is extended.
For already extended themes, we need a DB update.

The problem happened when loading the config.xml.
Bootswatch has an XML which could be thought as, somehow, non-well-formed XML (or at least a form of XML that is not expected or handled consistently by SimpleXMLElements).
I have seen this only in Bootswatch.

When there is a default value:

<options>
<cssframework type="dropdown" category="Simple options" title="Variations" parent="cssframework">
Default value
<dropdownoptions>
<option>Test</option>
</dropdownoptions>
</cssframework>
</options>

This is transformed to PHP (and then JSON), as

{options: {css-framework: 'Default Value'}}

When there is NO default value:

<options>
<cssframework type="dropdown" category="Simple options" title="Variations" parent="cssframework">
<dropdownoptions>
<option>Test</option>
</dropdownoptions>
</cssframework>
</options>

This is transformed to PHP (and then JSON), as

{options: {css-framework: {dropdownoptions: {option: 'test'}}}}}`

So, when there is no default value, the internal nodes (dropdownoptions) are intended to be included into the LSS.
writeXmlFromArray can deal with arrays, ints, strings, ... but is not prepared for stdClass.
That's when the XMLWriter fails.
https://github.com/LimeSurvey/LimeSurvey/blob/f62e9142e45b665796eec774a9368517161121f4/application/helpers/export_helper.php#L3110C28-L3110C38

Summing up:

• The problem occurred when loading the config.xml.
• To save the options in the database, they are converted to JSON, with the TemplateConfig::formatToJsonArray() method.
• We created a new method, that, when detecting empty nodes, traverses different through the elements, not allowing multiple levels of indentation.

This should also happen on Fruity, as it doesn't have a default value.
In the new fruity, there is a default value, so there shoudn't be hits on that theme.

Would love to listen to your thoughts before approaching the update routine.

Thanks

Thanks @gabrieljenik for debugging this.

If the "Default value" was stored in a tag or as an attribute, could the XML traversal be normalised?

Maybe Adam can review code ?

If the "Default value" was stored in a tag or as an attribute, could the XML traversal be normalised?

Actually, when storing as json on DB, LS code doesn't expect more nodes under an options node.
When there is "default value" text node, the nodes are somehow "lost in translation" and everything fits.
The problem is when there is no text node and hence the nodes under it are intended to be saved to the DB.

So I would store it as a an attribute.

Bootswatch has an XML which could be thought as, somehow, non-well-formed XML (or at least a form of XML that is not expected or handled consistently by SimpleXMLElements).

This part must be fixed too : we must use only valid XML, no ?
(Must create a dev issue)

I am facing a very similar issue. Exporting a survey using "fruity" theme works fine. If I then assign "extend_fruity" child theme, the export fails. -> Is this the same issue? Since at the issue description only Bootstrap is mentioned.

Is this the same issue? Since at the issue description only Bootstrap is mentioned.

Could be. I found stuff in the config.xml that resembles the conditions for this bug

@gabrieljenik how can I reproduce this issue?
I followed instructions in the ticket, but I couldn't reproduce this on latest master.
Is this only happening with MariaDB?

Seems @olle added a default value for the variation option on the Bootswatch config.xml on 19232 https://github.com/LimeSurvey/LimeSurvey/commit/eb27f86043718396dd71bcbde31b898683114615 .

That made the problem to be hidden.
To reproduce, please use the latest release (which still doean't have that update)

I can export surveys that use extended bootswatch and fruity themes.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=35781

Fixed in Release 6.3.8+231204

This bug seems to still affect version 6.4.5+240205

@bquiller I have just tested this using these steps:

1. Create a theme by extending the bootswatch theme
2. Create a new survey that uses the extended bootswatch theme
3. Export to LSS

and I managed to successfully download the lss file using LS version 6.4.5+240205.

Can you give me more details about your environment (browser, database ... ) and steps on how I could reproduce what you are seeing?