The thread dump won't help much here. Run a garbage collection afterwards and tell me what it reports (Tools->External tools->Perform garbage collection"). Try using another skin to see whether this is skin related. Look for images with extraordinary large dimensions. Try setting the image scaling quality to "Medium" to see if it's image size related.

Using Minimal, the error message appeared again, but all files were processed on the first run.
(I hadn't re-set scaling to medium)

Yes, that is what I did. Although I hadn't done anything else in jAlbum after the error message, I ran a new "Make album" and then the garbage collection.

But "Medium" scaling method seems to have been the fix.

Marked as answered

Yes, I am sure that MY memory is fragmented
I have forgotten how to access the -Xmx setting (although I am sure that I have accessed it in the distant past). Should it not be resettable from "Preferences" ?
And I had just updated to jAlbum 24.1 - shouldn't that have set it to -Xmx4000?

The image directory contains 66 items, totalling 422.2 MB

After restarting the computer, and setting the image scaling back to "Smooth", the problem returns.
After stating -
Processing image 39 of39 in VasseWetlands...Done
I then get -
An unexpected error occurred. Please see detailed description below:

java.lang.OutOfMemoryError: Java heap space
at java.desktop/java.awt.image.DataBufferByte.<init>(Unknown Source)
at java.desktop/java.awt.image.Raster.createInterleavedRaster(Unknown Source)
at java.desktop/com.sun.imageio.plugins.jpeg.JPEGImageReader.readInternal(Unknown Source)
at java.desktop/com.sun.imageio.plugins.jpeg.JPEGImageReader.readRaster(Unknown Source)
at com.twelvemonkeys.imageio.plugins.jpeg.JPEGImageReader.readImageAsRasterAndReplaceColorProfile(JPEGImageReader.java:498)
at com.twelvemonkeys.imageio.plugins.jpeg.JPEGImageReader.read(JPEGImageReader.java:388)
at se.datadosen.util.FileFilters.doLoadImage(FileFilters.java:735)
at se.datadosen.util.FileFilters.loadImage(FileFilters.java:657)
at se.datadosen.jalbum.AlbumBean$StandardImageProcessor.processImage(AlbumBean.java:6208)
at se.datadosen.jalbum.AlbumBean$StandardImageProcessor.processImages(AlbumBean.java:6069)
at se.datadosen.jalbum.AlbumBean$ImageProcessingTask.call(AlbumBean.java:6003)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)

My Exec line looks like this:

Exec=/usr/lib/jalbum/jre64/bin/java -Xmx1200m -DuseDesktop=true -jar /usr/lib/jalbum/JAlbum.jar

If the file isn't having permissions for you writing to it, just adjust that:

sudo chmod 777 filenameHere

I'm unfortunately not able to tell Java how much RAM its allowed to grab by any other means than the command line parameter -Xmx. If you're running jAlbum on Java 14 then the default amount of max RAM has been increased from 64MB to 1/4 of your computer's available RAM, I believe, so less need for that -Xmx switch.

Great!