Hi James,
Thanks for the bug report providing steps to produce the problem. I was able to replicate it here and fix the problem. It is fixed now in our source code repository and will be fixed in the next release.
For now there is a workaround to solve it, you can delete the file /Setup/ProviderConfig/indexbuilders/galleryindexbuilder.config
The bug was in a sql ce method used to select image gallery content for indexing in the search index. The only downside is that descriptions and captions of images in the Image Gallery feature will not be indexed in the search index until you get the real fix in the next release or from the repository.
When new items are published it triggers the indexing on a background thread and caused the error to happen.
Best,
Joe