I've upgraded to .161 and I see the same behavior.
Here's a part of the log started immediately before "build missing thumbs", any ideas?
Media Center; Version: 19.0.161; Types: 2147483647
1555125: 4636: General: CMCResourceHelper::GetIsModalPopupShowing: Menu showing
1556578: 4636: General: CMCResourceHelper::GetIsModalPopupShowing: Menu showing
1556594: 4636: General: JROptionsPageInfo::GetDialog: Start
1556594: 4636: General: JROptionsPageInfo::GetDialog: Options page: Tree & View
1556594: 4636: General: JROptionsPageInfo::GetDialog: Creating options page object
1556594: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Start
1556610: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Enumerated 58 modes, added 20 modes to array
1556610: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Finish (16 ms)
1556610: 4636: General: JROptionsPageInfo::GetDialog: Creating window
1556610: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Start
1556625: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Enumerated 58 modes, added 20 modes to array
1556625: 4636: Playback: CDisplaySettingsHelper::EnumDisplaySettings: Finish (15 ms)
1556625: 4636: General: JROptionsPageInfo::GetDialog: Updating window
1556625: 4636: General: JROptionsPageInfo::GetDialog: Finish (31 ms)
1559063: 4636: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
1560078: 4636: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
1561766: 4636: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
1561797: 4636: General: CMainUIWnd::Resize: Start
1561828: 4636: General: CMainUIWnd::Resize: Finish (31 ms)
1561860: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Start
1561860: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Filename: http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262178&Type=Full
1561860: 2124: Reader: CWinINetReader::Open: Start
1561860: 2124: Reader: CWinINetReader::Open: Opening http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262178&Type=ThumbnailsBinary
1561860: 4948: Reader: CWinINetReader::Thread: Start
1561860: 4948: Reader: CWinINetReader::Connect: Start
1561860: 4948: Reader: CWinINetReader::Connect: Finish (0 ms)
1561860: 4948: Reader: CWinINetReader::DownloadFromHTTPURL: Start
1561860: 4948: Reader: CWinINetReader::DownloadFromHTTPURL: Failed (header status code: 500)
1561860: 4948: Reader: CWinINetReader::DownloadFromHTTPURL: Finish (0 ms)
1561860: 4948: Reader: CWinINetReader::Thread: Finish (0 ms)
1561860: 2124: Reader: CWinINetReader::Open: ConnectToURL failed, m_bConnectError=true, URL=http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262178&Type=ThumbnailsBinary
1561860: 2124: Reader: CWinINetReader::Close: Start
1561860: 2124: Reader: CWinINetReader::Close: Finish (0 ms)
1561860: 2124: Reader: CWinINetReader::Open: Finish (0 ms)
1561860: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Finish (0 ms)
1561953: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Start
1561953: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Filename: http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262182&Type=Full
1561969: 2124: Reader: CWinINetReader::Open: Start
1561969: 2124: Reader: CWinINetReader::Open: Opening http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262182&Type=ThumbnailsBinary
1561969: 1804: Reader: CWinINetReader::Thread: Start
1561969: 1804: Reader: CWinINetReader::Connect: Start
1561969: 1804: Reader: CWinINetReader::Connect: Finish (0 ms)
1561969: 1804: Reader: CWinINetReader::DownloadFromHTTPURL: Start
1561969: 1804: Reader: CWinINetReader::DownloadFromHTTPURL: Failed (header status code: 500)
1561969: 1804: Reader: CWinINetReader::DownloadFromHTTPURL: Finish (0 ms)
1561969: 1804: Reader: CWinINetReader::Thread: Finish (0 ms)
1561969: 2124: Reader: CWinINetReader::Open: ConnectToURL failed, m_bConnectError=true, URL=http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262182&Type=ThumbnailsBinary
1561969: 2124: Reader: CWinINetReader::Close: Start
1561969: 2124: Reader: CWinINetReader::Close: Finish (0 ms)
1561969: 2124: Reader: CWinINetReader::Open: Finish (0 ms)
1561969: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Finish (16 ms)
1562063: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Start
1562063: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Filename: http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262194&Type=Full
1562063: 2124: Reader: CWinINetReader::Open: Start
1562063: 2124: Reader: CWinINetReader::Open: Opening http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262194&Type=ThumbnailsBinary
1562063: 5960: Reader: CWinINetReader::Thread: Start
1562063: 5960: Reader: CWinINetReader::Connect: Start
1562063: 5960: Reader: CWinINetReader::Connect: Finish (0 ms)
1562063: 5960: Reader: CWinINetReader::DownloadFromHTTPURL: Start
1562063: 5960: Reader: CWinINetReader::DownloadFromHTTPURL: Failed (header status code: 500)
1562063: 5960: Reader: CWinINetReader::DownloadFromHTTPURL: Finish (0 ms)
1562063: 5960: Reader: CWinINetReader::Thread: Finish (0 ms)
1562078: 2124: Reader: CWinINetReader::Open: ConnectToURL failed, m_bConnectError=true, URL=http://192.168.10.149:52199/MCWS/v1/File/GetImage?File=262194&Type=ThumbnailsBinary
1562078: 2124: Reader: CWinINetReader::Close: Start
1562078: 2124: Reader: CWinINetReader::Close: Finish (0 ms)
1562078: 2124: Reader: CWinINetReader::Open: Finish (15 ms)
1562078: 2124: General: CThumbnailCacheInfo::CreateThumbnail: Finish (15 ms)
[...]