Thank you for providing the URL to your gallery via email.
It looks like the data for your gallery's XML file is being generated correctly so all the correct files are in place and not corrupt.
However, the problem seems to be that the XML declaration in the dynamically generated XML file (the <?xml version="1.0" encoding="UTF-8"?> line) is not at the very top of the document. There is a blank line before the XML declaration which should not be there and this is very likely to be the cause of the problem.

I cannot be certain but I suspect this may be a bug in the version of PHP which is installed on your web server.
(What version of PHP is installed on your web server and has your web host recently upgraded the version of PHP or changed any PHP settings?)
Please try the following suggestion which should hopefully solve your problem:

Open the module's 'juicebox/includes/JuiceboxGallery.inc' file in a plain text editor and change line 171 from:

return $dom->saveXML();

... to:

return $dom->saveXML($juicebox);

Please note that the line number above refers to the recommended release of the module (dated 2014-Jan-28) which is available from this web page.

Something must have changed for this to happen.
If you did not change anything yourself, then check to see if any automatic updates to your Drupal installation were installed.
Also, check with your web server to see if they have changed anything recently (such as upgrading the version of PHP installed or changing any PHP settings) which may be contributing to your problem.

It would be much easier to troubleshoot your scenario if you were able to provide the URL to the gallery in question so that I could take a look for myself. Without seeing your gallery's embedding code, I have no way of knowing how the path to your gallery's XML file is defined.
It sounds like the PHP file within the Drupal module which dynamically generates the XML data may have somehow become corrupt or missing but, if this were the case, then re-uploading the module's files to your web server (which you have already tried) should solve the problem.

These files and folders are unique to each individual gallery and it is not necessary to copy them into the module's own file structure. (The module deals with these files and folders each time you create a gallery.) Only the 'jbcore' folder should be integrated within the module.

As we did not write the module ourselves (and the problem you are encountering seems to be related to the gallery's embedding code which is generated by the module), you might also like to post in the Drupal forum where the author of the module should be able to help.

However, if you continue to experience difficulties, please post back with the URL to your gallery and I will certainly try to help further.

If/when this happens again, please zip the 'presets' folder and email it to me (I have sent you a message regarding another forum query so you should have my email address) before you trash it so that I can take a look for myself and try to replicate the problem. Thank you.

Unfortunately, there is no quick and easy solution to your problem.

It is certainly not possible to change the way JuiceboxBuilder-Pro generates the SEO content code.

If the data you wanted to use for your SEO content code were embedded in the IPTC Document Title and Description fields within your images, then you could have JuiceboxBuilder-Pro use this information to automatically populate the image titles and captions respectively (using 'Images -> Titles' and 'Images -> Captions' from the drop-down menu at the top). The image titles and captions (sourced from the image's IPTC fields) would then be used in the SEO content code.

Perhaps you could find a program that would create a batch process to copy the content of the IPTC Keywords field into the Document Title and/or Description fields of all your images so that you could use this information within JuiceboxBuilder-Pro.

Otherwise, you could perhaps use a plugin (for example for Lightroom or WordPress) to create your gallery and then modify the plugin to generate SEO content code.
However, this would likely be a complex and time-consuming project.
You would need to:
(1) be familiar with the structure of the plugin you intend to modify.
(2) be familiar with the programming language used by the plugin (e.g. Lua for Lightroom or PHP for WordPress).
(3) create a method to iterate over the images in your gallery.
(4) extract the information you want to use for your SEO code from the images.
(5) output the SEO content (using the information extracted from the images) as HTML code to the web page.

How this would be achieved would differ greatly between plugins but the principle (the five steps above) would remain the same.

No. It is not possible to tell JuiceboxBuilder-Pro what data to use to populate the tags and attributes in the SEO content code.
JuiceboxBuilder-Pro will always use the image title for the <img> tag's 'title' attribute and the image caption for the <img> tags's 'alt' attribute (and will also use both the title and caption as text within <p> tags).

This CSS issue has now been fixed in v1.4.0. Please see this blog entry for details.

This CSS issue has now been fixed in v1.4.0. Please see this blog entry for details.

This CSS issue has now been fixed in v1.4.0. Please see this blog entry for details.

Thank you for reporting this bug.
It looks like the problem is caused by the combination of setting both useFullscreenExpand="TRUE" and expandInNewPage="TRUE". I have logged a bug report with the developers.
Until the bug is fixed, a workaround would be to set one (or both) of these configuration options options to "FALSE".
As your gallery currently has a transparent background, you could perhaps set expandInNewPage="FALSE" and then set an opaque background color for your gallery in expanded mode, for example expandedBackgroundColor="rgba(0,0,0,1)".

I have been unable to replicate the problem you have reported.
Using the current version of WP-Juicebox (v1.4.0.0), I can display a Juicebox gallery sourced by your Picasa Web Album images using the username and web album name that you do.
I see no problem with your gallery's configuration option settings.
Try re-uploading the WP-Juicebox files to your web server to ensure that all files are present and correct.
If you are in any doubt as to whether or not the existing files are being overwritten, try deleting them from your web server first before uploading the new files.
Also, you could try to display a different Picasa Web Album using the username 'picasateam' and the web album name 'VegasWeekend' to see if this sample web album displays OK.

Unfortunately, it is sometimes necessary to change the 'theme.css' file between versions (when bugs are fixed or new features are added). We are aware of this and have tried to make configuring a gallery as flexible as possible (for example with the available configuration options and the use of vector icon fonts) without the need to modify CSS (although I understand that some things may be achieved only with custom CSS).
Perhaps a solution to your problem would be to have your modifications in a separate CSS file and use the !important declaration to ensure that they override the CSS rules from the gallery's own 'theme.css' file.

Juicebox does, indeed, still support older versions of Internet Explorer. IE8 still has a small but not insignificant market-share and we do not want to alienate users by dropping support for older browsers too early.

There is only one version of Juicebox available and, therefore, I am unable to supply you with a version available which does not include support for older versions of Internet Explorer.

Juicebox v1.4.0 has just been released and introduces a new expandedBackgroundColor configuration option so it is now possible to set different background colors for normal and expanded modes. Please see this blog entry for details.