Cannot add Gallery; “An unexpected error occurred….”

Have read similar postings with similar errors but not found the problem.

If I go to “Add Gallery/Images”, enter a new gallery title, then drag an image (35k size jpg) it is listed in the filenames area. I click “start upload” and get an immediate error popup:

“An unexpected error occurred. This is most likely due to a server misconfiguration. Check your PHP error log or ask your hosting provider for assistance.”.

No directories or files have been created in “galleries” folder.
If I upload the same image to Media then try “Import from WordPress Media Library” and create a gallery, I get an “Importing Gallery” progress bar that hangs.
If I enable php logging, I see no errors. File permissions on the wp_content folder are 755.
I have tried resetting the options under “other options” to no effect.
After all the above, gallery folder is still empty.
If I go to “manage Galleries” and use “add new gallery” then I can create a gallery and a corresponding folder appears in the file system of the same name with a thumbs folder in it.
If I now click on “save changes” I get an error thrown “Class DOMDocument not found”
If I return to “Add Gallery/images” and attempt again to upload the image I get the same “An unexpected error…..” message but now the image is in the folder, however I can do nothing with it. If I return to “Manage Galleries” and try “Scan folder for new images” I again get the error thrown “Class ‘DOMDocument’ not found”.
Run out of things to try; any suggestions?

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