Have you tried recovery option "Scan and rebuild scripts, layouts, etc. "?
It seems there is hidden layout that closing layout group, appear in recovery log having name "--".
On your file, maybe a layout group is deleted but the closing layout remains accidentally.
What version of FileMaker?
Thank you for your replies. This is currently a version 11 system and I had tried a recovery process in v11 without success.
Subsequent to your suggestions, I have just done a test conversion to .fp12 format and ran a recover in v14. This was successful. The offending "</Group>" was no longer in the xml and the xml document validated correctly.
Incidentally, a recovery with v13 worked too.
Many thanks again for your assistance. The issue is resolved.
As background, it is good to keep in mind that Layout Folders were introduced in FileMaker 11: Tony White Designs: FileMaker 14 Code Objects as ERD
I had a similar issue a while ago and got a deep dive answer from Lisette Wilson of Informing Designs, which I am posting below with her permission.
I once had a similar issue with corruption in the Script Catalog, where scripts could not be moved within the list. In that case, I always had a recovered script named "--" in the recover.log file, and one other script that would be recovered. The "--" script showed up in the recovery log even after I deleted all scripts. (From a copy, of course.)
What eventually resolved the issue was to remove all script folders and dividers, then open the file in a version prior to the advent of script folders. At that point, I was able to delete the troublesome script. When you open a file with Script folders in an older version, the folders are represented by scripts with the "--" name and a Script with the folder name. I suspected that after the scripts were organized into folders, someone using the old version, deleted one of the scripts marking a folder, but not all pieces of that folder.
Perhaps something similar has happened with Layout folders in your case.
I was able to remove those scripts in 8.5, then open it in 11 and get a clean recovery. At that point, I just repeated the script removal with 8.5 on the original database, as rebuilding from scratch was not an option.
Hope that helps.