If you need to rebuild thumbnail cache for some reason you need to delete the MediaInfo.db file in AirVideoServerHD configuration folder.
Since Server 2.1.0-beta2 the visiting the following URL (on the computer running Air Video Server HD) should clear thumbnail cache
For older server version:
1. Open (Applications / Utilities / Terminal)
2. Copy the following command and paste it to Terminal
rm ~/Library/Application\ Support/AirVideoServerHD/Data/MediaInfo.db
1. Open Command Line (Start -> Search for or Run cmd.exe)
2. Copy the following command and paste it to cmd.exe (text can be pasted to cmd.exe by right-clicking in the window)