I offer some ideas I have applied to recover corrupted files (not just Scribus).
First make a backup of the errant file, as an insurance policy.
Next download and install XMLCopyEditor a free XML editor.
https://sourceforge.net/projects/xml-copy-editor/With the original document file safely backed up, open a copy of the *.sla document in XMLCopyEditor.
Since *.sla is really *.xml format you should be able to inspect the inner elements of your file.
There are two buttons in top menu showing "yellow ticks".
Check well-formedness
Validate
Check first that the document is "well-formed" (that is, it follows syntax rules).
Check next that the document is valid.
If one of these fail it may be that the xml syntax is corrupted.
It then becomes a process of trial and error to eliminate corrupt elements in the xml document.
I use a blank template Scribus document as a reference for comparison. That is, a Scribus template without content in pages.
You can also compare Scribus XML documents (old and current document versions) using meld.
https://meldmerge.org/========================================
[P.S] I see another related thread here which might offer more recovery ideas (such as splitting up the source document):
http://forums.scribus.net/index.php/topic,3655.15.html========================================
[P.P.S] Another idea is to comment out blocks of xml elements in the test *.sla document to identify which page or attributes or corrupt syntax might be at fault.
More found here ..
https://stackoverflow.com/questions/2757396/how-do-i-comment-out-a-block-of-tags-in-xml