Hi Kirk,
Eventually, the Catalog became accessible again. I turned on Catalog debugging and proceeded to remove more images from my catalog. Every time, the catalog stops responding (search/browse/filter/collections do not respond) for an amount of time seemingly proportional to the number of images removed, and CPU usage of pm-task shoots up during that time. It takes minutes to remove hundreds of images, and I cannot keep working during that time.
The logging does not offer much. Many rounds of very slow image removal resulted in just this:
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Photo Mechanic Plus version 6.0.0.0 build 5215 (abbcb1f)
[Wed Oct-21-2020 11:15:32.997][10f2955c0][5066036] PMDocumentController.mm:2140 macOS version: 10.14.6
[Wed Oct-21-2020 11:15:33.067][10f2955c0][5070204] PMDocumentController.mm:2145 Current locale is: C
[Wed Oct-21-2020 11:15:35.658][70000f6d9000][5154316] [00003ffe1f4ae6d0] Initializing DNS resolver
[Wed Oct-21-2020 11:15:37.046][10f2955c0][5206912] PMCache.cpp:3180 cache: main thread startup
[Wed Oct-21-2020 11:15:37.046][10f2955c0][5207040] PMCache.cpp:1382 cache: spawning 4 worker threads (4 cpus) ...
[Wed Oct-21-2020 11:15:37.071][700012b84000][5215724] PMCache.cpp:3120 cache: found 1216 disk cache entries, 1216 usable
[Wed Oct-21-2020 11:15:37.072][10f2955c0][5273240] PMCache.cpp:661 PMCache::PMCacheWorker::set_memory_cache_size_limit_mb(1638 MB) [1717567488] bytes
[Wed Oct-21-2020 11:20:39.041][70000f8e6000][7095556] [00003ffe20107274] PMCache.cpp:1959 cache: prune oldest image info: 24448 30319 35606 43292 61572
[Wed Oct-21-2020 11:29:35.588][700016399000][7201680] NRFGIF.cpp:492 NRFGIFLoadImageScaled(): Width or Height of GIF image too small, GIF error code: 0, file:'/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/dot.GIF'
[Wed Oct-21-2020 11:29:35.589][700016399000][7201680] NRFBitmapBase.cpp:689 NRF failed to load '/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/dot.GIF' with error 5
[Wed Oct-21-2020 11:30:04.461][700016399000][7215496] NRFGIF.cpp:492 NRFGIFLoadImageScaled(): Width or Height of GIF image too small, GIF error code: 0, file:'/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/borderfade_h.GIF'
[Wed Oct-21-2020 11:30:04.461][700016399000][7215496] NRFBitmapBase.cpp:689 NRF failed to load '/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/borderfade_h.GIF' with error 5
[Wed Oct-21-2020 11:30:04.463][700016399000][7206300] NRFGIF.cpp:492 NRFGIFLoadImageScaled(): Width or Height of GIF image too small, GIF error code: 0, file:'/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/borderfade_v.GIF'
[Wed Oct-21-2020 11:30:04.463][700016399000][7206300] NRFBitmapBase.cpp:689 NRF failed to load '/Volumes/Photos/2005/050508 Wetlands Project/Web/galleries/res/borderfade_v.GIF' with error 5
[Wed Oct-21-2020 11:40:50.074][700012b84000][7641500] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:50.074][700012b84000][7641700] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:50.416][700012b84000][7643320] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:50.416][700012b84000][7643320] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:51.905][700012b84000][7583584] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:51.905][700012b84000][7583700] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:51.914][700012b84000][7584724] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:51.914][700012b84000][7584924] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:52.589][700012b84000][7446064] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:52.590][700012b84000][7446456] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:52.593][700014b90000][7447092] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:52.594][700014b90000][7447676] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:52.596][700014b90000][7447676] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:52.596][700014b90000][7447676] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:55.780][700012b84000][7414288] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:55.780][700012b84000][7414288] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:56.940][700016399000][7416956] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:56.940][700016399000][7416956] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:40:57.206][700013b8a000][7417564] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:40:57.207][700013b8a000][7417564] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:41:01.137][700015393000][7434424] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:41:01.142][700015393000][7434424] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID
[Wed Oct-21-2020 11:41:03.170][700013387000][7424920] ColorManagementCommon.cpp:21 LittleCMS error: 9, 'Wrong input color space on transform'
[Wed Oct-21-2020 11:41:03.170][700013387000][7424920] ColorManagementCommon.cpp:466 CreateColorManagedBitmap(): no transform found for image to profile ID