Huh...I have the exact same issue as @zeoboromir, but when I look inside the zip file (downloaded today from here) I do not see ThirdScreenSupportLib.dll in any of the subfolders at all. I turned off chrome safe scan in case that was deleting the file or something, but same result. Am I even downloading this from the right spot? Apologies, this is my first run-through getting my arcade cab built and setup.
\ThirdScreen-v6.0.1\Plugins contains:
ThirdScreen.dll
ManagePlatformVideoMarquees.dll
\ThirdScreen-v6.0.1\ThirdParty\MediaInfo\x64 contains:
libcurl.dll
MediaInfo.dll
libcrypto-3-x64.dll
libssl-3-x64.dll
(x86 contains variants same as above....but i don't see "ThirdScreenSupportLib.dll" anywhere)