Some images visable...some not. All show up in slideshow.

Gallery version = 2.3.1 core 1.3.0.1
API = Core 7.54, Module 3.9, Theme 2.6, Embed 1.5
PHP version = 5.1.6 apache2handler
Webserver = Apache/2.2.3 (CentOS)
Database = mysql 5.0.77, lock.system=flock
Toolkits = ArchiveUpload, Exif, Ffmpeg, NetPBM, ImageMagick, jpegtran
Acceleration = none, none
Operating system = Linux dom0 2.6.18-164.15.1.el5xen #1 SMP Wed Mar 17 12:53:17 EDT 2010 i686
Browser = firefox

I think this problem came up after an upgrade and I never got back to it.
Some images don't show up when browsing the gallery. I noticed that if I do a slideshow that they do show.
I see this over and over in my http error log.
PHP Notice: Undefined index: /mnt/media/g2data/locks/20622 in /var/www/html/gallery2/modules/core/classes/FlockLockSystem.class on line 188, referer: https://secure.sitename.com/gallery2/main.php?g2_view=core.SiteAdmin&g2_subView=core.AdminMaintenance

I notice when I try to rebuild thumbnails the following output.
Debug output for failed items:

file_exists(/mnt/media/g2data/cache/entity/2/2/22957.inc)

(mysqli): SELECT g_id, g_entityType FROM g2_Entity WHERE g_id IN (22957)

(mysqli): SELECT g2_Schema.g_info FROM g2_PluginMap, g2_Schema
WHERE g2_PluginMap.g_pluginId = g2_Schema.g_pluginId
AND g2_Schema.g_type = 'entity' AND g2_PluginMap.g_active = 1

(mysqli): SELECT g2_DerivativeImage.g_width, g2_DerivativeImage.g_height,
g2_Derivative.g_derivativeSourceId, g2_Derivative.g_derivativeOperations,
g2_Derivative.g_derivativeOrder, g2_Derivative.g_derivativeSize,
g2_Derivative.g_derivativeType, g2_Derivative.g_mimeType,
g2_Derivative.g_postFilterOperations, g2_Derivative.g_isBroken,
g2_ChildEntity.g_parentId, g2_Entity.g_id, g2_Entity.g_creationTimestamp,
g2_Entity.g_isLinkable, g2_Entity.g_linkId,
g2_Entity.g_modificationTimestamp, g2_Entity.g_serialNumber,
g2_Entity.g_entityType, g2_Entity.g_onLoadHandlers FROM g2_DerivativeImage,
g2_Derivative, g2_ChildEntity, g2_Entity WHERE g2_DerivativeImage.g_id IN
(22957) AND g2_Derivative.g_id=g2_DerivativeImage.g_id AND
g2_ChildEntity.g_id=g2_DerivativeImage.g_id AND
g2_Entity.g_id=g2_DerivativeImage.g_id

file_exists(/mnt/media/g2data/cache/entity/2/2)
is_dir(/mnt/media/g2data/cache/entity/2/2)
file_exists(/mnt/media/g2data/cache/entity/2/2)
is_dir(/mnt/media/g2data/cache/entity/2/2)
getParameter permissions.file for core plugin
getParameter permissions.directory for core plugin
rename(/mnt/media/g2data/cache/entity/2/2/22957.incSjuyBr,
/mnt/media/g2data/cache/entity/2/2/22957.inc)
file_exists(/mnt/media/g2data/cache/derivative/2/2/22957.dat)
file_exists(/mnt/media/g2data/cache/entity/2/2/22942.inc)

I'd appreciate any help you could give.