XMPMetadata installed. Issue with Web hanging up, and refreshing metadata fails after a while with a page not found. Many errors in apache log, and debug log.

Remove, and all is well

• is xmpmetatdata required to use XMP sidecar files?

It is always a good thought to look at the plugin documentation. For XMPMetadata:

"Extracts XMP metadata from images and XMP sidecar files.
Author: Stephen Billard (sbillard)
Enable this filter to scan images (or xmp sidecar files) for metadata.

Relevant metadata found will be incorporated into the image (or album object). See Adobe XMP Specification for xmp metadata description. This plugin attempts to map the xmp metadata to database or IPTC fields.

If a sidecar file exists, it will take precedence (the image file will not be examined.) The sidecar file should reside in the same folder, have the same prefix name as the image (album), and the suffix .xmp. Thus, the sidecar for image.jpg would be named image.xmp.

NOTE: dynamic albums have an .alb suffix. Append .xmp to that name so that the dynamic album sidecar would be named album.alb.xmp.

There are two options for this plugin

The suffix of the metadata sidecar file
A list of image file suffixes that may contain metadata
Check each image type you wish the plugin to search within for an xmp block. These are disabled by default because scanning image files can add considerably to the processing time.

The plugin does not present any theme interface."